Welcome aboard by the way, i forgot to say that, its my age you know

Originaly the diff would have had a tag attached to one of the housing bolts, the tag would have had the ratio marked on it, but inevitably the tags vanish over time. As with most (all?) Ford diffs though, they do take the time and trouble to stamp the ratio in the gear, but that means pulling the plate off the diff to get inside.
See the attached pic to tell the difference between the 2 different diff sizes.