I know there is probably a disparity between NOS and production line parts (if nothing else, a time difference), but you can compare these two, a 7173 '68 and a 7708 '69. Both are 20-25 years old, and both are stamped in about the same location. I'd have to dig my original '69 out of the archives, but I don't remember a stamping on the face of the balancer.
Casting marks on the back are similar to the previous descriptions. Only the offset and the stamping (if they are) really set these apart.
I second the previous motion - buy it (especially if he's discarding). You can always have it rebuilt -