I don't know of a specific serial or date range. I do know that early on the PM45's seemed to have a few issues to work out. One reason I waited to buy mine, I finally gave in and went for it with no regrets.
I think the new one or probably all within the last year or so are pretty solid performers. We do still hear of a trigger bar issue from time to time but no so much on the 45 but accross the board.
If you got your eye on one especially if it's new I'd not hesitate to go for it. If by some poor misfortune you get one with issues kahr will take care of it, if it's minor issues theres lots of help and smart folks right here to help you work it out.
I'm fondling my PM45 with much affection as we speak.
In Memory of Paul "Dietrich" Stines.
Dad: Say something nice to your cousin Shirley
Dietrich: For a fat girl you sure don't sweat much.
Cue sound of Head slap.
RIP Muggsy & TMan
"If you are a warrior legally authorized to carry a weapon and you step outside without that weapon, then you become a sheep, pretending that JOCKO will not come today."