Scubaroo once bubbled...
...it moved... bludgeon it again.
I finally got the Christolube in the mail to finish rebuilding my TX50. When I put the o-ring on the venturi, I defied the instructions, and installed the venturi into the body by itself first, rather than putting the venturi over the valve spindle, and installing it into the body that way. Initially the new o-ring was still showing, but now that the reg was clean (and there was no sand behind the o-ring!), I was able to "click" the venturi further into the body, completely hiding the o-ring. I then inserted the valve spindle through the venturi into the body. This appears to be how the reg was designed to be assembled, but is slightly different in the documentation. Doing it as per the rebuild docs would have resulted in the o-ring showing again. With the venturi clicked fully in, the o-ring is hidden, no grit can get behind it, the lube is protected, etc. The venturi still functions as normal.
So the solution may be as simple as rebuilding in slightly different fashion at service time - insert the venturi into the body followed by the valve spindle, rather than insert the assembled valve spindle/venturi into the body.