[John Sabato]

The rear tab should be a "perfect" fit or as close as you can get it by polishing the tab until it will enter the slot without much resistance. The front tab if too thick should be polished on the underside until the takedown lever will close, again "without much resistance. It is a "feel" kind of thing. The sideplate when fitted properly should not "float" between those two points. If it does, you will never get a consistent trigger pull.

The curve on the front of the sideplate should be a matched fit to the diameter of the takedown lever.

Now you know why these pieces were numbered to the gun when they left the factory. Other "may" fit, but only the original parts were manufactured to fit when the gun was presented for the final acceptance inspection.
You said that you had a sideplate and a Luger that were a perfect fit... use it as your control when polishing the new sideplate. They should feel pretty much the same when fitting is completed.

In addition, the hole in the trigger lever should be a good polished fit to the pin that holds it. If it is a sloppy fit, then again, the trigger pull will never be consistent, but will vary as the sloppy lever gets pulled down by the trigger. Polish all bearing surfaces of the trigger lever (not metal removal mind you, just polishing where the pieces may drag when rubbing against each other.)

The result should be a reasonably good trigger pull.