I think the hold open feature will be lost. The original PP mag has a metal follower and the left side at the front is all the way against the inner edge of the left side of the mag body. Placing a P22 cut off mag in there shows that the top of the stock mag definitely won't catch. I can cut that portion off and reshape the top of the follower as necessary....but with the follower inserted into a P22 mag, shoved into the PP magwell....it doesn't look like enough polymer engages the stop arm to last for long. Shooting will tell if I get that far.

The metal band down the center of the rear of the mag is 0.006" thick at present. That is thin enough. No more off the rear of the mag. More can be taken off the barrel for the mag spring...but I've already taken off a lot. The mag fits but tightly. It needs to be a bit looser. The magwell might benefit from a good cleaning but all in all it is already in pretty good shape. I need to remove the grips for a better look at fitment.
The original mag is just smaller. Rear width of the mag is 0.43" and tapers inward toward the center of the mag immediately. The P22 is 0.444" and does not taper. I have not thinned the mag along the sides. Perhaps they are dragging on something inside the grips.
The round portion of the original mag that allows the full circle on the front of the follower measures 0.435 at the bottom, 0.429 at the top. And varies along the length slightly between the two. The P22 barrel measures 0.442 at the bottom and 0.439 at the top and varies along the length. At the base of the mag well a stock mag....bottom and top can be stuck in at an angle and there is plenty of room. You cannot align a stock mag with the proper rearward slant and insert it though. Too wide front to rear. When I look down the magwell from the top as the loose at the bottom mag moves up it becomes tighter and tighter in all dimensions. Possibly the magwell tapers but the original magazine is not tight at any point and with no slide installed will rattle slightly in the magwell. Tricky Germans.
You can see the cutout on the left side of the original mag so that there is no interference with the ejector. Note the metal follower reaches all the way to the left and if you look really closely you can see a tiny white area which is where there are light scratches from the bottom leg of the ejector when engaged by the follower. Note how narrow the top of the P22 follower is and that the left, front portion of the mag will likely interfere with the ejector. It should be easy to make it match the original somewhat.
Both mags seem to hold a round at exactly the same height with regard to the rear of the mag and at the same slant. The rim will be released just a bit later due to the longer rear lips on the P22. The larger difference seems to be the front section of the lips. The nose of the breech rail is tapered a bit inward and should have no problem engaging the base of the round. Height when seated will be determined by the top of the cutout for the mag catch. More can always be taken off...none can be put back.
In this photo I aligned the nose of the rounds with a square. You can see the position of the original catch cutout as it will sit above the expanded tabs on the P22 mag. The tabs will be removed.
Rear view.
Original follower engaging the ejector lower leg.
The P22 follower is iffy, even iffy with regard to wear of the steel leg against the polymer. It isn't an essential feature...if you don't have a mag for your PP anyway. I won't know how it works until a properly fitted mag is installed and test fired.
I'm not sure how the P22 mags are actually made. Stamped, crimped, folded but a center strip seems to be welded??? in some manner down the center. IOn the right is a stock mag and on the left one that has been filed and polished down to 0.006" at the center strip which is the thinnest point. I can't twist the mag but there is no benefit to further removal of material.
The mag slides easily in except for the last 1/2". It can be pressed all the way up but should not require any additional effort. Something is fitting too tightly but it doesn't appear to be at the bottom. That portion still wobbles a bit. I'll figure it out tomorrow. 1917