Interesting one that Dave, copy of running instructions I have is dated Dec
1939 and covers Mar-Vil and Century "Mark 4 models with gravity feed tanks". This must have been quite a late introduction as a spare parts list dated Jan 1938 covers only the suction carbs (three different types).
As for which engines should be correctly referred to as Mar-Vil and which as Century - I would say that all the smaller engines are Mar-Vils (remember the cylinder dimensions inherited from the Johnson Utilimotor) and the larger versions are Centurys. However that is just my personal opinion and unless we can dig up any contemporary Villiers marketing material we cannot be sure.
The parts list again covers both, adding a "C" suffix to those parts unique to the Century. It is interesting to note that among the parts common to both engines is the base tank and it is of course this part which usually bears the Century or Mar-Vil designation.