IMI is the original, the rest are just "licensed" copies. I have an original, model "B".
There is nothing wrong with the copies, as long as they are stamped correctly, all critical aligned, and all holes mounting points correct. Hide in a box, fire both in sucession, and ask the bystander which was which. They never know.
Supposedly the Group/Vector frames have a heat treatment to make stronger, do not know if this will really make it better than the IMI, as maybe Group started with a lower grade steel? We'll never know, heat treat may discount this concern.
I read an article or blurb in MGN I think it was where the issue of heat treat was a concern between the IMI SMG and the IMI semis on the market. I wrote Class Three Supply, ad the fella wrote back indicating his inside source at IMI stated that all IMI receivers got the same treatment. I called Stan Andrewski (fun guy to talk to),, he asked how old I was, told him, he said keep shooting, you won't wear it out in your lifetime.