I am suspicious that there may have been machine oil in that area,, just a drop,, just enough that a patch over fails as well.. hard to know at this point as I have completely gutted the mod now--- attempting to Frankenstein him back to life now.
On the Frankenstein issue for others attempting this,,, this mod is cast,,, which includes a a block/shelf that supports the 510 fixtures,,, it needs to be removed for most replacement 510s. We used a small reciprocating saw (normally used on trim work for us)--- metal blade.,, This removes most of the material,, I am spending more time on this than you would expect,,, but thats because I am trying to use parts / parts of my bone yard rather than buy new fittings.
Concerning 510 fixtures to buy,, with this mod I would have to concentrate on skinny lock nuts / area,,, this wouldn't be an issue if the cast design wasn't there at the 510 location- I would have been done hours ago! -- no biggie,,, watching tv & messing with this is good learning experience for a completely dif diy mod I want to make.