Vintage work crew photos

[Toolmaker51]

With a frame like that I'm wondering if that just hydraulicly "squeezes" the rivets to form them?

Well, not in that era, but close.
By the mid-60's, a process named "clinching" had been developed. Clinching joins sheet metal by drawing and forming the part materials into an interlock...eliminating welding and fasteners...no sparks, fumes or heat.
https://btmcomp.com/clinching

[Frank S]

And most certainly not this far back