It's a 1935 Mk.I, and this 'ole has been bugging me for not a little while.
I've now at last got down on knees and did my best to photograph the outlet.

This is the offending orifice in the passenger's floor board:

There appears to have been a cover of some sorts.

From underneath:


And again:


Visitors have suggested various uses, some very imaginative indeed.
Apart from the one that seems to immediately spring to every visitor's mind, I'm at a total loss to explain this away.

Any suggestions anyone?

Geoff - Let's see who's the most imaginative
Edit: 1 changed to the proper I

Location: South Norfolk - Next to Suffolk

Thats easy, its so the passenger can tak a leak without stopping.

To aid communication, when smuggling an illegal immigrant into the country? He might even be able to keep an eye on the speed, by looking up the spout.

Shove a broom handle down it, as an extra handbrake on steep hills?

Lazy passenger can pass spanners/munchies to the driver/mechanic, whilst he is working underneath the car?

It looks pretty useful. I might add one to our Box.

Location: Towcester

Looks about the right size for hiding a bottle when pulled over by Mr Plod.

Location: Herefordshire (with an "E", not a "T"!)

Failed attempt to bring heat from the silencer into the car.

En-suite.

But that is some considered modification, welding etc.,
so not a whim, but what for?
Oh yes, that's the OP's Q.
Presumably this would normally be under a rubber mat or carpet?

Location: On the very Edge of Europe....

OK Geoff - here's my sensible guess...

I think the driver was a very keen smoker, that had an asphalt ash tray.
The matt or carpet would have needed a hole (as Sandy points out), but this person was obviously very committed to his habit.

I look forward to hearing a better idea.

Colin

Location: Towcester

I reckon its an overflow when driving through floods.
Stephen

Location: Colchester

Put a skyrocket down the tube, light the blue touchpaper and there you have it - a rocket assisted take off device.

Tony.

Location: Malvern, Melbourne, Australia.

The pipe is reminiscent of a self bailer used on sailing dinghies, by being angled down into the water they create a vacuum as the boat moves forward and suck any water out of the boat.

I wonder if its an attempt to pull more heat through from the engine bay. Are there any extra holes in the bulk head?

Stuart.

Location: Devon

Its an emergency braking system !
This is what you do, secreted along side the passengers seat is an old broom handle. When the passenger sees the driver stand on the brake pedal and try to wrench the hand brake out of the floor whilst using that old expresion "Oh s---t"
He takes the broom handle and shoves it down the hole untill it makes contact with the road thus bringing Ruby to a standstill in a few hundred yards or so!!
Simples

Location: As far east in kent as you can get

No extra holes other than the normal.

Geoff - Keep the ideas coming, they are wonderful.

Location: South Norfolk - Next to Suffolk

If you put a long stick through the hole from underneath what does it hit on the bulkhead area?

Location: United Kingdom

It's for catching the drips when topping up the gearbox oil...I would have thought that was pretty obvious!

Location: Near Bicester.

I suspect that if the end of the pipe is near to the silencer, warm(ed) air might be drawn up into the footwell of the car, either that or deadly exhaust fumes!
It could be a 'Q' modification to allow a pre-war James Bond to empty a box of tin-tacks onto the road to puncture the tyres of a pursuing car?
There again, would a pre-war James Bond have had an Austin Ruby? I believe Ian Fleming had his famous character at the wheel of a Bentley.
I can't honestly say I wish my SWB saloon had a similar facility!

Perhaps an attempt to use this principle to remove fumes from a smoking engine from the car.

It is a top secret military modification .
As I have signed the Official Secrets Act , I am not allowed to divulge any more information !!

Sorry , Merv

Location: New Forest

It seems to me that this is a very early attempt at "in car" entertainment. If ever there was the obvious equipment to play in car "splat the rat" this is it. The small piece of broom handle - sorry - auxillary brake is in fact the rat splatter. You had to keep the kids amused somehow and having your 5 year old slung between the chassis members during play was a great way of keeping them happy - (and if you tell the young folk that today, they won't believe you!!

Its for night driving. A broom handle or large mechanics screwdriver is placed down the hole and when the vehicle is straddling the cats eyes it will twitch allowing the driver to follow the road despite the non existent headlights. A further benefit of this ingenious system is as a speed measuring device, knowing the spacing of the cats eyes allows the driver with a suitable stopwatch to calculate his or her speed.
Steve.

Location: East of Sandy and Brian but still not quite in England

I'm still puzzled...

One Saloon A7 I had in the 60's had tin cans around the exhaust pipe and a joint to bring the hot air into the car ... not a good idea in summer I thought. But it was October when I bought the car and I drove it from Winchester up to Arbroath in one go, no motorways them days... I appreciated the warmth.
Somebody then offered me folding money, which was the same ££'s as I paid for it, plus a few dollars more AND the coach fare back home - Sold!!
I only had the car 4 days.

Location: On the very Edge of Europe....

That would be my guess too, maybe with something like this sat on top
http://www.prewarcar.com/index.php?option=com_caradvert&view=ad§ion_id=3&id=92289

Well known NZ born international motoring writer Eoin Young in his youth possessed a Seven. It was fitted with a Model T coil and a plug in the silencer. He would backfire the car in front of the milkbar crowd and drop assorted engine parts through the floor!

(In explanation. For decades NZ had 6pm pub closing and 21 year drinking age. For many youth milkbars provided the social meeting place.)

Bob Culver

Location: Auckland

And thus the HAPPY HOUR in Oz....
I seem to remember in the 60's it was 7pm closing in Brisbane, so on the way home from work,
call in a bar, throw as much as possible down and then go home....
BUT perhaps it was 6pm...

The bars I recall both in Brisbane and Sydney where just white tilled rooms,
totally inhospitable and devoid of Sheila's.
Cairn's was just

But perhaps Tony Press may have more accurate details of the bars at that time
- I was in no fit state to be objective....
Great holiday though, one of my mates decided to stay, he's in Hobart now,
though how he's still alive I've no idea.

Location: On the very Edge of Europe....

Sandy,

You accurately describe 'the 6 o'clock swill' - taps were turned off at 6 so before that you lined them up on the bar and went your hardest.
I think drink driving was not such a crime as now

The women were sedately drinking their Advocar and Cherry Brandy? in the 'ladies lounge'.

Tony.

Location: Malvern, Melbourne, Australia.

Or even Advocaat and Cherry Brandy

Tony.

Location: Malvern, Melbourne, Australia.