Yes, I'm with Mark on his valuation. Now that it's a 20 it is "plain jane" and loses the extra rarity of a 21. And the weight isn't so light for a 20. Still nice though. And don't feel bad about the 21 mis-measure. I once bought a 21" A Zildjian which arrived and was a 20. But it was so nice I wasn't worried. Mine is 1975g and late 1950s.

Later 60s maybe not so sought after as early 60s (although I don't know what proportion of potential buyers would even make the distinction).

And the rivet holes in a non standard pattern don't help the price. Rivet holes may not lower the expected price much either. They probably aren't a big factor.

So lighter the better for older Zildjians?

With vintage cymbals, thinner cymbals tend to be much more prized than heavier ones. Really heavy ones don't tend to sing and have the best combination of sticking and wash and sustain that I, for one, am looking for. There are always exceptions like a fairly heavy '60s Istanbul K Zildjian 20" that Steve Maxwell is selling. Here is a link to that cymbal from Drumaholics recent thread.

http://www.maxwelldrums.com/zildjian-istanbul-1960s-cymbal-2313g-p-9960.html

I think the reason that lighter is more prized is because cymbals have been manufactured heavier over time. You can buy a heavy modern ride cymbal all day long but lighter modern rides are not as common. Many of us vintage folks prefer the wash, complexity, and subtleties that a lighter vintage cymbal usually provides.