I have two of the pre magnum frame 640s and they are my favorites of all J frames.
One is a first production run from '89 with the CEN prefix serial number. It's got the best action from the factory of any of my J frames and my new production no lock 642-1 has an Apex Tactical trigger kit.
The other is a regular production from around '94.
I used to have an NYPD spec one from around ''91 but foolishly got rid of it. It was slicked up by an NYPD armorer and was very nice. The seller told me he used it once in the line of duty and would only say it was a good day for him and a bad day for the skell.
Get the pre magnum frame in my opinion. They balance better for me. I'd also never fire magnums in a J frame, too punishing.