$650 for a 70's JM in excellent condition would be a steal.
To put things in perspective, the current "Remlin" guns with ugly cross bolt safety are selling for $900 at Wantsalls and $930 at Wholesale Sports. The older ones are generally better regarded (the mythical JM stamp undeniably adds market value, though I would argue it's unwarranted superstition) by many folks.
If the gun is really nice, on the EE here, I think $800 or so is realistically what it could sell for without having to bump it a bunch of times. I think the seller could even coax out $900 if they wait a couple of weeks for the right buyer. These guns are really sought out by the CAS crowd.
If he takes $650 though, WIN!