Observations on two model 645's

[AZ Shooter]

Since I bought one of the "early" ones with the phillips screw in the decocker and a "later" one with the hex screw, I have noticed several other differences.
First, the backstrap on the early gun is metal while on the later one appears (and feels) to be plastic.
Next, I notice that the early gun has a "half cock" notch on the hammer that the later one does not. I have noticed that if, after decocking, the hammer is manually cocked to the notch, the DA trigger travel is reduced by about a quarter of an inch. To me this shorter DA pull seems to be an advantage for first shot placement.

 

[AZ Shooter]

Since I bought one of the "early" ones with the phillips screw in the decocker and a "later" one with the hex screw, I have noticed several other differences.
First, the backstrap on the early gun is metal while on the later one appears (and feels) to be plastic.
Next, I notice that the early gun has a "half cock" notch on the hammer that the later one does not. I have noticed that if, after decocking, the hammer is manually cocked to the notch, the DA trigger travel is reduced by about a quarter of an inch. To me this shorter DA pull seems to be an advantage for first shot placement.

 

[Outrider]

My understanding is the 645s had some such merits that disappeared with the 4506s -- and for that reason, some guys still prefer the 645.

I don't know firsthand, though.