yep thats an oldie for sure
but at this point you have a couple choices
1) use it "as is" and hope osha doesnt come around and see it
2) buy a new s/s

if its that old then maybe its time to replace it?
they arent really all that expensive compared to the fine you'd get from the dataplate missing.

besides, do they even supply parts for it anymore? consider that as well :o)

if you're just wanting the capacity of the truck with the sideshifter on it that should be on the truck's dataplate.

I also found a neat little tool on cascades website, a capacity calculator which can be found here:
https://w w w.cascorp.com/web2/orders.nsf/americas-en-capacity!OpenForm
(just remove the spaces from between the w's)

may that might be of some help
but in the end you might be better off just getting a new s/s.