Good evening Reckless. From your photograph I think that you have Lucas L131 headlamps which were not introduced until the introduction of the Ruby in 1934. The give away is the more pointed (bullet) shape whereas the RP should be fitted with the LB130 which has a more rounded back.
Brian is quite right, the hole in the back of the n/s shell is to take a shouldered countersunk screw which supports the solenoid dipping mechanism.
I am intrigued that Brian's RN is so fitted as one of the strengths of the RN was the fitting of Lucas 35/32 headlamps with the 'Lucas Graves' double filament bulbs which as Dave Mann is on record as saying elsewhere is infinitely superior to the 'n/s on o/s off' dipping arrangement of the Ruby.
Regards
Stuart

Location: Staffordshire - the creative county

Stuart, my RN is fitted with double filament dipping bulbs but they are in much more modern reflectors. Perhaps it was once fitted with solenoid dipping ones but the car has obviously been messed with over the years.

Location: Near M1 Motorway Jtn 28