The mags for the older P89s came with a more elongated hole for the mag catch. IIRC, the top of the hole was rounded rather than the rectangular hole in the newer mags. Someone here posted some pics a few months back comparing the old mags to the new mags.
I have an older P89 (can't remember the serial number, I'm at work and the P89 is at home) that was made before the switch. However, it came new from the factory with the newer mags and all the newer mags I've tried work just fine in mine. I think it was forum member Coffeepot (who used to work for Ruger and, again IIRC, was one of the engineers who designed the P89) that said that, for the older P89s before the mag change, newer style mags will usually work 90%+ of the time in the older guns, but as always check your individual gun with new magazines. According to what I remember, if a mag works, it was good to go; if it didn't work, then the individual mag was incompatible with that particular gun.
For replacement mags, MecGar flush-fit 17rd mags have always worked VERY WELL for me, as have factory mags. The only aftermarket mags I've tried were ProMag; as ab4ka stated, avoid like the plague. Turned my stone-cold reliable P89 into a jam-a-matic.
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