I would NOT reuse either part.... 18 mm is a lot larger than 5/8" so I don't think you could mismatch those.... Regarding rethreading something that is 18 mm x 1.25 mm to 18 mm x 1.50 mm, the peak of one thread would meet the top of another in only 3 threads of engagement.... completely destroying the thread.... so again, no way would I try and use that from a strength point of view, even if it "doesn't leak".... I agree that the regulator was glued to the tank, as the O-ring recess around the top of the threads is full of ground up grains of a greyish material (JB Weld?)….Bob
Threads at top doesn't look the same to me as at the bottom. Plus, the raised ridge of metal (arrows) would have happened as something was threaded INTO the bottle, not out of it IMHO.
From the looks of it, I would say imperial threads in to a metric bottle. Bottle looks like 18x1.5mm in the end and something else in the beginning. Regs and bottles are marked for threads due to specifications, so take a look if they are not a match. Marko
Just so you know, there is nearly 1800 lbs. of force trying to blow apart an 18mm thread at 4500 psi.... Proper engineering practice is to make sure it won't fail at over 3 tons of force.... Please be safe !!!Bob