very interesting!!! It does look to have a filament, but hot-cathode argon tubes run very very hot... I would assume that the tube would need cooling, but I have seen older and DIY argon tubes (pulsed and very low power) which only use passive cooling, but also dont have a filament. I'd love to get my hands on that to find out what it is!!. Do you have a HeNe power supply?? if you do you can apply it to the tube, that will at least make the plasma glow slightly and tell us what it is. The little ring in there appears to be a 'getter' of sorts...
I would also say it's a HeNe but the mirrors are throwing me off. They look clear. It could be that this is the plasma tube and it utilized external mirrors to lase. I'm so curious now!!
Very neat!