John,
I agree with you that many of the earlier and pre 1948 734 mowers would be assembled in Manchester, the later red and cream models I believe were from Kilmarnock.
The one I have was bought new in 1952 from Grounsell and Sons, M-H agents in Louth, as you say that is very late for this type of mower. I have heard the stories from their old lorry driver who used to take the Austin Loadstar lorry all the way to Scotland and bring a load of machines stacked high back to Lincolnshire, he told dad when it was delivered new he had collected it from Kilmarnock only a few days previously.
Malcolm.