I see on some threads the reference to 3rd generation pistols (so I assume there would be a 2nd and 1st gen). Is there any way of telling what gen my .45 is by it's serial#? And does it really mean anything ?
Are you asking about generations of your M&P pistol? I'll try giving you an answer until someone more knowledgeable shows up.
"3rd Generation" refers to a series of metal framed semi-autos like the 3913 and others, which aren't in production any more, as far as I know. Different series of S&W revolvers have been marked as "686-#" for example, to identify features changed or added since the last number series.
I don't know that the M&P pistols have any different "generations" or "dash" series...yet. So, to my limited knowledge, the M&P pistol is still on its first generation.