Hi jon of its a 3.54 then its a defender / disco / classic rangey diff, does it have a filler plug? is it definately A 3.54 as they were never fitted to series motors so someone must have fitted it later? They fit no problem...
It's a 3.54 - I bought it for my series, but after I'd fitted it I realised that something wasn't right - pulled it back out and counted the teeth and worked the ratios out.
It should have a filler plug on it - I managed to fit it on the back of a series and get oil in it somehow I'll double check before you collect if you are still interested.