could it be Reynolds High Manganese tubing? I had an EF Russ that was apparently this (the tubing sticker said it was, but I think it may have been repainted so couldn't say for sure). From what I remember the weight was slightly heavier than a 531 frame of it's size, so that would seem to match yours frames weight
I guess it must have been whatever was good quality tubing at the time - it had been repainted before I first saw it, and as far as I can remember there was no indication under the paint.
could it be Reynolds High Manganese tubing? I had an EF Russ that was apparently this (the tubing sticker said it was, but I think it may have been repainted so couldn't say for sure). From what I remember the weight was slightly heavier than a 531 frame of it's size, so that would seem to match yours frames weight