I know there a at least a couple of ways of naming the differences between the different stages of development. Basically they are all Mk1 body shell with alterations to mechanicals, trim etc

(with of course 3/4/5 door, 1.7/1.4 and /2 litre variants)?
Model year is the best way to tell them apart!
A quick look at the range of Ford Fiestas 1976 to 2011on wikipedia throws up the same problem with face lifts in the later models especially and it all gets very confusing
In fact Mk1 and Mk2 Fiesta are really the same chassis. The problem crops up again later with later Mk's. on the 90's cars.
Here in Little Britain a Mk1 300 means the nose cone/first headlamp type, Mk2 is the thin filler strip above the front bumper and Mk3 the bigger bumpers with no strip. Not a perfect division at all and as Anjo has said can cause problems!
I know this has come up a few times before but it's important to people who are working on older or near changeover cars!