Hadn't thought about the pins for the side clip. I'll have to think about that a bit.
I haven't had much time to work on the sheetmetal mockup of the mount bracket, but here's a picture of what little progress I've made. Both sides are the same at this point, but will be different after cuts are made for the ejection port, assembly pin, and so forth. I've also got to allow for access to the serrations on the rear of the slide (wouldn't do much good to get a better sight picture, but not be able to chamber a round). The straps running down each side of the grip are only there to help hold the mockup in place for ejection tests, and are not intended to be on the machined version. Since I'm using the TLAR method, the mockup might end up being made 3 or 4 times before I'm happy with the clearances on the various cuts. But it'll probably be a few months before I have a mill, so I've got plenty of time to get the mockup perfected.
I've seen similar marks on spent casings from my P3AT, but haven't noticed any marks on the slide. Doing a little testing with ejecting an unfired round, I noticed that it throws the round to the right. Holding the grip horizontal, with the slide to the left, the round ejects upward (which would be to the right, if it were held normally) and slightly to the left.
This might not be as difficult as I was expecting it to be.