The parts are the same size but the notches (where the spring goes) are slightly offset from each other. The notch on the sp1981 is about 1/2 the notch width further towards the rear of the catch than the 1035

That makes sense (although I wasn't expecting it to be as close as it is) with some visualization. From the point of the spring, the larger frame is larger in both directions whereas the extended mag release only increases in one direction.

 

For a Gen 3 G17 I like the Vickers mag catch.
Its projection from the grip it is between OEM standard and OEM extended which I think is perfect. I find the OEM extended to be too long.
https://ghostinc.com/vickers-glock-tactical-magazine-catch/

 

For the Gen 3 Glocks the Vickers Tango Down extended mag release (same one that Ghost sells) is some of the best $16. you can spend for a Gen 3 part. It's longer than the OEM mag release, but shorter than the OEM extended mag release. But what makes it better is it's rounded edges. Both the Glock OEM parts are sharp and will leave a mark.......

 

You could round (file) off the sharp edges of the OEM extended, but I'd still prefer the slightly shorter Vickers (Tango Down) mag release button for the Gen 3.

 

Thanks for the tips. I got the parts (wanted to stay "stock") today, and used some really fine sand paper to round off the corners of the extended mag catch. At the same time, I ordered an extended slide stop.

What's the deal with the different finish on Glock pistol parts (specifically, slide stops) and Glock spare parts? This is the third time (at least recently) I've bought what's marketed as a Glock brand slide stop to replace a factory one. In each case, the original pistol part had what seemed more like a semi-gloss black paint finish. Each of the spare ones looked more like it'd been parkerized. Really rough matte black finish. I used a cotton patch with some dried flitz on it to smooth the finish on the new slide stop before installing it.