Regarding MIG (wire feed) welders,
I got even more curious and read up on when these consumable electrode wire welded came into being and learned that while it did exist back in the late 40's early 50's, the shielding gases were still very expensive. Also the process was still not refined as far as voltages and how the power supplies worked. MIGs didn't get popular in manufacturing until the 60's.
And as for rivets, hawk I concur as my ol' '50 still had round rivets holding the front bumper on, but I had to cut them off so I could hammer the bumper off the Jeep to get it somewhat straight. I was a little wistful as I was cutting them... I may use some "faux" rivet/bolts when it comes time to put it back on.