You are right of course, Mike should buy an oem as long as they are available, but in my own defense, my 2 Motobecane model 7,s were given to me including the brass adaptors and a gas (natural) valve assembly which was way too big and heavy to fit and I had one of the small brass in-line valves that I used on a remote gas bottle for tuning. Having these pieces in hand, it only took about 15 minutes to cobble a pet cock that looks and works well and can be replaced at the local motorcycle shop, if needed.
I am guessing that the brass adaptors were a pipe thread which approximated the metric thread in the tank closely enough to screw into the captive nut without binding.
Did I mention that I cut down an aluminum bicycle pump to fit the frame pegs like the original. Oh... the chrome plastic door protectors at the auto parts store are an exact replacement for the chrome trim on the frame down tube. I almost forgot, I also sized my own main jets using a clock bushing tool to reduce the size and a Honda condensor wired into the harness at the coil to replace the bad oem on one of them.
After 2, 200 mile round trips to moped army BBQ,s, the bicycle pump worked perfectly for roadside tire repair and the petcocks work perfectly.
I don't have an in line gas filter, I acid cleaned the tanks and rely on the screen in the carb to keep the big stuff out.
By the way, which shop are you now working for?