Interesting, I've never come across one with that little filament, must keep my eyes open wider. Shame about the lack of etch, sadly it happens all too often with older lamps

Philips made a few oddball blacklight mercury lamps back in the days, like this tubular HPW-T 80W, which was for some special lab applications only (it has a phosphor coating too and never made it into regular production).

That’s a fascinating UV lamp Max! Saying it also has a fluorescent coating makes me think its an off-cut from the Black light fluorescent tube variant where the low-pressure discharge in predominantly short-wave UV and the coating convert it to longer wave UV. I believe this isn’t required in high pressure lamps as the discharge contains more long wave radiation anyway. I’d be interested to know if this improves the UV emission for this lamp.

High-pressure mercury arcs do emit some UV-C light, not as much relatively to their input power as low-pressure mercury discharges, but it's there and it's not negligible. Philips did sell a phosphor-coated HPW lamp featuring a higher UVA output, so there certainly is a use for that shortwave radiation. The HPW-T 80W shown above is very peculiar indeed, especially given its evacuated outer jacket which enables an operation over a wide input power range topping at 80 W (judging by the burner size and electrical characteristics). Decreasing the input power shifts the peak spectral emission of the mercury arc towards shorter wavelengths, and reduces the sleeve's temperature too. So, there must be some power settings where a significant part of the lamp's UV output comes from the phosphor. Some more detailed analyses are certainly needed to better understand how this lamp works exactly. This is for sure a very interesting engineering piece in my collection.

Tremendous, to have such an unusual lamp in your collection.