Soviet Union stamps - UV paper

Greetings,
Something new to study after a summer - Soviet Union stamps on fluorescent paper.
 One might think these are not very uncommon considering the printing numbers and stamps issued.
But it doesn't appear to be so once you take a closer look.

What renewed my interested after a long time was recent auction of David Feldman. There, a pair of mint 6kop definitives, 1966 issue,  was realised for 130 Euros. I hadn't put much time in examining my stockbooks and thought this could be a worthwhile adventure. So with a UV lamp in one hand and tweezers in other, I dived into the albums in my darkened room ...

I did find 4kop of the same definitive series on UV paper but not 6kop. Unlike other stamps on UV paper, this one glowed quite green. Sadly, I know of now way to show my discovery.
 Today I took another look at this subject, after being reminded by a friend collector to check Zagorsky catalogue for reference. On a sidenote, Soloviev did list 4kop on UV paper but somewhy for nearly the same price as ordinary paper.

After examining catalogue and my stockbooks, I found atleast two early issues on UV paper.
Both from 1965. One is Antarctic ships, 4kop se-tenant (printed together in a pair). Since I got a set of these in quarter blocks from other source recently, I could confirm the stamps I have are not on ordinary paper.
Second one is Heroic City Odessa. Both are listed in Zagorsky. Unfortunately atleast in 2006, there are no prices for UV papers.
Third discovery is our own Estonian theme, song festival. It would appear all the stamps I have of this issue are on UV paper.

This seems quite interesting field to pursue. Downside is that it's real difficult to show your finds. You also need a portable UV lamp and a dark room. On upside, this subject has a lot of potential in the future in my honest opinion. Anyone can discover such stamps from their collection if they only bother (discovering them at collectors fair or via eBay isn't possible though, unless you buy them from a trustworthy source). They ain't too common I'd guess but there are more possible paper varieties of this kind out there than Zagorsky lists.
 And most UV papers ain't listed in any major catalogues at all. One could easily build a specialised collection of these far before any specialised catalogue with prices is published, making it a good investment as well as a fun subject in this interesting hobby.

Regards,
Toivo