I finally got to own one of these rare birds after several failed attempts to get one! This bulb is going to be well protected! Its a vintage Sylvania 100w H38-4HT lamp from around '68. NOS and all.
Come to think of it, I was born in the 70s and while I certainly remember BT25 mercs and HPS lamps from way back even a few cleartops, I don't even recall seeing a single clear 100w mercury BT25 in use!
Thanks all for the comments! I have some classic Hi-Output White /W mercury lamps that should be on the doorstep anytime this week. GE, Sylvania, Radiant, and even a Norelco.
Update! I am now the owner of not one but two BT-25 clear mercs, it is a Sylvania just like this but is slightly used and it came from a different seller.
All the Best
Colin