Paul,
A few years ago,a customer, had me remove a lamp,for him for replacement,from a fitting in a shop, this was an internally-silvered lamp, with the bottom part of the bulb glass silvered, upon withdrawing it, I noticed it was not a standard lamp, it was a BC22 type base, but it had Three location pins on the sides of the brass cap, spaced at 120 degrees,looking at it from the top.
Can you or anyone else please tell me, how
common,this type of lamp configuration is?,
as I had never seen it before then,nor have I seen it since.
Strange, but true. [Linked Image]