The P17 needs 4 total o-rings. 1 piston seal, 1 breech seal, 1 valve plug seal and 1 valve stem.  This kit includes enough o-rings for 3 rebuilds.

Beeman P17 O-ring kit. You will receive:

3 piston seal o-rings. Standard size,  If you keep going through these o-rings, deburr the air intake hole
3 breech seal orings, slightly oversized, should work better
3 valve plug seal o-rings. Smaller than the included breech seal o-rings, but can also be used as a breech seal.  Doesn't fail as often as the other orings. 
3 valve stem o-rings. (tip of the valve stem) if you hear air escape at the end of the cocking stroke, this might be the problem

I have a video on YouTube called:  "How to Disassemble/Teardown Beeman P17, Replace O-rings, Reassemble and more"
https://youtu.be/1ECdfEwrZlQ

Will be shipped in a stamped envelope with no tracking for both domestic and international.