Cue eBay!
This is a loose stamp that turned up on eBay. Note the postmark! Here's a close-up of the handstamp:
Sure enough, an unrecorded type with wide space between the last zero and the word "man". To add further glee to what was actually a really good eBay week, the same seller also sold me this stamp:
NOT Tiflis Vokzal but the Tiflis-Dzhulfa TPO serial "zh" The handstamp?
Wide space between last zero and "man"... So I'm going to assume this is the fifth value of this group of handstamp types, since the link with Tiflis railway station is reasonably clear.
Who knew? Frankly, it makes me wonder what else is lurking in the (by now ridiculously large) number of Georgian handstamp issues I haven't looked at yet...
LATE UPDATE. As an illustration that I really should take a better look at the stamps I already have, it turned out that my hoard of used 20.000 stamps also contained a example of the Tiflis Vokzal type: