I did notice by the pictures they showed on Ebay that one of those probes had a larger orifice between the o rings than the other, I think you ended up getting the one with the small orifice (approximately the same size as original by the photo). The one that has the larger orifice also comes with a spacer for using on the other SPA models like the M16 but it's just so easy to make your own spacer if need be. I'm tempted to buy one exclusively for my M16 so that way I wouldn't be replacing o rings so often on it. My probe still has the original o rings on it.
FYI, the tendency of the probe to eject from the rifle is because one O-ring is sealing better than the other. Inspect them closely with a magnifying glass or simply swap their positions.[edit] I just re-read and see where you tried replacing the O-rings. That may indicate a slight defect in the groove nearest the end that is making it hard for the O-ring to seal when pressure hits it. Since you have a replacement that works well, I suspect this info is of no particular use to you but I wanted to comment in case someone else is having a similar issue. [/edit]
That is a great looking probe. Does anyone know if this also fits the PP700? I'd like to eventually get a stainless steel one for mine - the brass one is holding up OK so far, but I do see the stress marks building on it.