(also my first post) The car is most likely a MKI as the MKII Mexico didn't come out until early 1976. In general the last MKI's were N reg but as with all cars when a new model arrives it can be hard selling off the old one so afew lay around on dealers forecourts for a while before being registered for the road I have come across afew MKI's that are P reg i've also seen afew MKII's on an X reg.

The best way to get information on these cars is to join an owners club the Sporting Escort Owners Club cater for all models of MKI & MKII Escorts there is also the RS Owners Club and the AVO Owners Club.

http://www.seoc.co.uk/

http://www.rsownersclub.co.uk/

http://www.avoclub.com/

(hope the information isn't too late as you posted this in September)