Ian,

Top left was purchased last year, bottom right is original.

Hope this helps your quest.

Thanks Ruairidh, checking a half dozen original plugs this morning, have found a very slight variation but not significant enough to take full advantage of the threads available in the Gbox.Also checking five boxes four had five threads and one only three due to different suppliers maybe.
Thanks David, in order to use two fibre washers at 1/16" thick (which most originals seem to be) plus the brass cover, a length of 7/16" protrusion would insure that we could catch all available threads with certainty, clearance from the gears didn't visually appear to be a problem with the quick check this morning.
Using thinner fibre washers is a help but still falls short in terms of threads used and the increased chance of pulling the bottom threads from the Gbox. Cheers Ian

Location: Bristol

And another Ian - I am almost certain this (steel) one was produced by the Austin Seven Centre many years ago.

This has appeared on eBay;
http://www.ebay.co.uk/itm/Austin-7-Seven-brass-plug-/252834335731?&_trksid=p2056016.m2518.l4276

Location: Near M1 Jcn 28

That's interesting Brian, looks like a casting rather than a machined one off, also looks a more reasonable length. Anyone have any idea if someone stocks them? Ian M

Location: Bristol

I am fairly confident that the one in the ebay link is a blanking plug for plumbing or similar.

I thought the proportions looked odd especially the hole in the middle, but put it down to the camera angle.

Location: Bristol

Location: Huncote on the pig

A7 Services makes sense Tony

I have a couple of the steel ones. They certainly came from Chris Dalby a long, long time ago.

Steve

Location: North Yorkshire

Interesting point Ian, all the presumably original gearboxes I have stripped that still have the brass plate have a thin paper washer between the plate and the gearbox and a thin(ish) fibre washer between the plate and the plug. Sorry, at work at present so can't illustrate the point :(
Anyhow, these seem to leave sufficient threads in engagement to prevent problems.

Location: North Wiltshire