Proportioning valve plug

I posted yesterday about swapping my master cylinder to the cobra style. Well I took the old master cylinder off, and was confused as to why there was already an adapter on it. It looks like whoever did the disc brake conversion already put on an SVO master cylinder. Decided to pop off the proportioning valve plug to see what they did and it’s already been gutted, and the plug was just filled with RTV. I don’t really feel comfortable putting this back in so does anyone know what size and thread pitch it is so I can put in without a hole in it? Also, they didn’t put in an aftermarket proportioning valve, no wonder my brakes were shit.