This is pretty darn heavy stuff. Despite the smaller than average overall size, it is extremely heavy. Frying pan grade cast iron I would say, LOL. Bruised the heck out of me carrying it out to the back yard. Considerably heavier than repro ones you buy now, but I can't claim to have picked up and compared anything from the 50's/60's. I'll weigh mine and report back.
Also, the legs detach from the chairs, if that is any help and they are held on by large bolts/nuts, through holes in the castings. These are probably replacement bolts, but I can't say for sure on that, they are frozen in place, rusted/painted solid. Boy it is hard for me to get a sense of age when it's been heavily repainted. I'll try to get some closer pix up in a couple of days. There are no markings on it anywhere. No numbers, no nuthin'.