711M just means that it was built after 1971

691 is the old version meaning it was built in 1969
686 is from 1968

The 1600 crossflow was fitted in a number of cars:
Mk1 Mexico
Mk2 1600 sprot
Mk2 1600 Ghia
Earlier Capris
Mk3 Cortina

Decent running ones go between £80 - £250 for a stupidly priced one from some greedy person but they are getting hard to find these days