What they are trying to tell you is the gun is considered a 3-screw. The mainspring strain screw is never counted.
Strictly the M&P Airweight and models 12 to 12-3 are built on the KA frame. While the A does designate Aluminum, there are also dimensional differences between the K and KA.
Ahh. OK, Thanks. Yes, it would be a 3 screw Airweight.