How interchangeable are parts in the two? I heard that some internal parts are so I was wondering about the back straps. I can't find any readily available for the sw99, however I found plenty for the p99. Before I buy them, I was wondering if anyone had done this before.

SW99 back straps can be found at

ww.midwayusa.com/find?sortby=1&itemsperpage=24&dimensionids=17482&pageNumber=1

smm1099,
What size are you looking for (small, medium or large) and I'm assuming you are looking for the full size in either 9mm or .40? I have some extras and might be able to help. :-)

It's full size 9mm. It's not a large strap as it doesn't go all the way up the grip, however I don't know if it's the small or medium.

I am looking for the medium backstrap for the full size SW99