After having read through this thread and have looked at these images I'm guessing $125-$150 USD max. Sixties' A's are not that rare or that valuable. I've got a couple I've bought for under $100 within the last year or two..
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.