Try a post on a the Capri owners club site or something (if you have not already)
Already did that thanks. They said
"Hi bb,
HYF is the correct engine code for a 3,0 litre V6 138hp (Model EV30H) as fitted to the Capri (HYB would have been from a Granada, but the engines are pretty much identical)
The location is the correct location for the engine number, it has been stamped pretty crudely, but it seems to read PU81857, which would make it production month JUNE 1974."