Scubapro Mk25 DIN knob with O-rings

In the picture of the Scubapro kit above you see there are THREE O-rings:
1) On the top, the small one. this is the one working on "modern" SP regs, such as the MK10, MK15, MK20 and MK25 (I am a piston guy...)
2) The second is just after the screw. This one is working on the old MK5.
3) The last O-ring, close to the plastic knob, is just a retainer for the knob, and does not make any sealing...
So, if you own a "modern" SP reg, you can get rid of 2). If you have an old MK5, as me, you should get rid of 1), possibly replacing it with a brass washer, 2mm thick, for ensuring the screw does not goo too much inside the reg body.