This is just a glass housing. It's (was, in pre-SMD times) pretty common for diodes. The fact that it's labeled "V" is a bit confusing.
I've got three possible explanation:
- Varicap: a diode used in non-conducting bias. The bias voltage determines the capacity of the isolating zone in the diode
- Voltage Reference: A Zener used as voltage reference
personally, I tend towards the second. But: with more info on the purpose, we might say more.
But @Trevor made an important point: older techs tend to misname "diodes" with "varistor". My guess is that this is what happened here, since all your diodes are labeled V. So, V6 is some small-signal diode in a glass package. That's all we can possibly say about it. There's thousands of different diode types that were sold in glass packages.
Even the one three-pinned device V2 and V5 are labeled V – although every bet would be they're transistors. I'm frankly beginning to think the names on your board don't actually correlate with the type of component...
The marking on the board indicate it's a polarized component. The D might be a good indication it's also a diode.
Given there's a weak correlation between device type and letter used, it's probably just some specific diode. Again, without your own measurements, we can't guess much.