no dataplate?
serial numbers are usually stamped on the frame of the truck in a couple of different locations depending on the year and model.
older model S and H series units were usually stamped along the right hand side of the frame on the outside edge along where the floor plate is, you could also pull the floor plate to uncover where it might be stamped and covered up.
Then later on they moved the serial number stamp back along the right hand side of the hood area on top of the frame, so raise the hood and look along the edge where the hood sits on top of the frame area, probably along back where the overhead guard leg is.
I've also seen it stamped into the fender or along the front panel RH side between the mast and truck.
If the truck has been painted theres a chance it was painted over and not seen easily so some scraping might be in order to reveal it.
Older model serial numbers usually started with a letter followed by 6 numerical digits like so Nxxxxxx (example), later on they changed their serial number layout which is what you see now a letter, 3 digit numerical , a letter, then 5 or 6 numerical digits ending in a letter.
would look like this E177NXXXXXXL (is just an example)

just fyi, you wont find it stamped on the mast, those numbers are mast group numbers and other pertinant info for the mast only.