It says Sylvania VT-229 on the base. Assuming it hasn’t been re-based I’d guess that’s what’s inside the glass envelope. VT-229 is the military coding (Valve Transmitting) for a 6SL7GT http://www.r-type.org/exhib/aaq0432.htm.
Looking at your three Trigon boxes I see two are labelled 6SN7GT and one 6SL7GT. The latter box has a 6SN7GT alongside it, suggesting a mix-up at some point. I’d venture that the Sylvania valve belongs in the 6SL7GT box.
I can see the shadow on the table-top of some lettering on this. A closer look at the lettering might reveal something about the manufacturer.
Trigon these days is a brand name owned by Edicron. They sell the Reflektor valves branded/printed with Trigon on them… Nothing wrong with that, they’re good valves. £10 each used might be a bit much for those particular items. When I have them they’re typically £7.50 rrp