Thanks Mike. I took a chance and used my 3/8 NF tap through and it worked great. Then I took a file to the steel weight as I don't have a 3/8 NF die, to clean up the damaged thread on the end of the weight and **magic** everything works like it should!
I picked up this gun yesterday. It's a Model 41, made in the early 60's, with both the 7 3/8" barrel and the 5" barrel, two mags, the aluminum and steel weight, compensator and end cap, and to top it off, the original box! Gun isn't mint but it's pretty close. Now I need to know which of the other 3 Model 41's needs to go to make room.