The production run took less than 2 months. GM (as any auto manufacturer would be) was certainly equipped for fabrication of this sort of pressed, crimped, spot welded, diecast product. LB made just short of a million No.4s in 4 years, in comparison.
There was a batch sold by Century in Canada years ago. While the price here has been rising, they are worth much more in the US. Read a test report a while back. At very close range shot well enough, recoil was noticeable.
They seemed like a good idea to someone at the time. When it came right down to it, they weren't really all that useful.