Marcus ,
At that time the stamping of numbers was not very deep into the metal of the chassis, so even if it has not rusted, a few coats of paints will cover the shallow marks very easily indeed.
On my two of my +4s the number was stamped on the top of the crossmember on the driver's side (on a RHD UK car) but both were very faint and I had to search a long time to find them. On the other one it had been re-chassied so many times that the original number had long been lost, and like Tim suggests, I bought some stamps and did my own. If after searching really diligently for a long time , you really don't find it, then follow Tim's advice and do it yourself.
