here's a sale of one where they say the edition was limited to 375 -
http://www.ebay.com/itm/LeRoy-Neiman...-/201472995682
Here's another one and the number, it says, is 32/375 - on the front
http://www.julienslive.com/view-auct...GNED-SERIGRAPH
Here's another one with no mention of a limited edition, although it is a signed print
https://goldinauctions.com/lot-9829.aspx
The limited edition seems to be priced in the thousands of dollars range and the last one sold for $316
My guess is the sticker on the back of the one you own and in the auction from February was not put there by Neiman but by the gallery - and it was not a limited edition - although I don't see any mention of a second edition on Nieman's website
http://www.leroyneiman.com/product/g...oncos-classic/
maybe you could contact them