When I look at the size markings on these sockets,
they look identical to those on Plomb sockets [and
wrenches, for that matter].
That distinctive font was continued to Proto tools, as well.
I've seen these and always passed them by because
1. Thickness and 6=point broaching mean weak steel to me.
2. Fugly, heavy, nameless orphans.
OTOH might be some early attempt at heavy duty or impact use.