Czechoslovakian Stamps: Any Rare Varieties???

I've just finished going through a very large lot of Czechoslovakian stamps and I was surprised to find, at least according to the Scott catalogue, everything is very low CV with no varieties or errors to look for. I've checked ebay sales history and it does seem that, with the exception of some of the very early issues, such as the Hradcany series, and some of the 1938 German overprinted issues, no Czechoslovakian stamp has ever sold for more than a few dollars.

So does anyone who uses Czech-language or other specialised catalogues know of any interesting varieties, such as colour errors, that would be worth keeping an eye out for?

Some Czechoslovakian stamps exist either normal vs. fluorescent paper; sometimes there is quite considerable difference on catalog value. For example with 1971 postage dues the 5.40K stamp has got minimum catalog value on normal paper, but fluorescent paper specimens catalog at 25€.

-k-

Interesting - I often forget that fluorescent paper issues are important varieties in many European countries.

Under UV light, I find there are three distinct types of fluro reaction. Do the catalogues distinguish between the ones on bright white paper and deep and bright yellow?