I picked up a finger extension for one of my mags (10 round 9c) and it doesn't fit. The attachment rails are different. Are the 10 round mags finger extension different than a 12 rounder? It's has the S&W logo on the bottom.

Yes the are different, the 10rd mag tube is shorter on the M&P 9c.

Did you order direct from S&W? I did, their .PDF parts list did not specify the part number (9mm mag base, flat) I ordered was for 10 round 9mm mags only. I just sent 2 back to S&W for exchange yesterday. Either their part number is wrong or someone has stocked the wrong parts in that bin in the warehouse.

Ok, thanks, I thought they must be different.

Great. I just ordered 3 from Midway 2 days ago.