I have both and have not really looked that close but for all I know the only difference is the 645 is DA (first shot) and the 745 is SA. Oh, I think my 745 IPSC has a blued slide too. Before I got my 745 I saw a single action 645 listed on GB. The seller's story was that in the first year of 645 production, before the 745 was available, the S&W Custom Shop would convert a 645 to SA only. Anybody know if this is true?
I've read about SA 645's, but the ones I've read about the work was done by gunsmiths - no mention of S&W doing it. That's a good question. Does anybody know if S&W did that conversion?