I have two just like this I got a couple years ago on eBay.

Cool !

Made in April 1979. I also have this lamp, possibly from the same e-bay seller, and this is quite an interesting model made only from 1977 to 1980. Its thick phosphor coating consists of a blend of yttrium vanadate and magnesium fluorogermanate. The later was added not only for its deep red fluorescence, but for its absorption in the blue, which is needed to lower the color temperature of the emitted light to 3000 K. By the way, its light output is not 5000 lm, but 3750 lm (at 100 h), which is quite low, even by the late-1970s standards. Sylvania phased this particular design out in 1980 when the company introduced the superior yttrium vanadate and yttrium aluminate garnet phosphor blend (then the output of this 100 W BT25 lamp rose to 5000 lm).

I love these little BT Mercs - They are so cool 😎
You can never have enough of them ,

they are the potato chips or cats of the bulb world, you can't have just one !

Cool find! I have two identical ones too. Very thick phosphor and dim too.

Thanks !