Apparently standards mean that the same motor frame doesn’t necessarily fit in place of another motor with the same frame! Typical bureaucratic paper shufflers! Anyway, the problem is that the old motor doesn’t have a registration, and the new motor does (as the standard says it should – see dimension N).
So I didn’t bother fitting the motor to the lathe. I was going to remove the registration on the new motor and fit it, but as I’ve got plans that need another spindle motor anyway I decided to just re-fit the old one (Yes, I’d taken the old motor off before I realised the difference!) and save this motor for the mill spindle that I’ll add the Denford. I am thinking of loosing the whole structure below the lathe bed anyway so that I can make a rigid connection between the mill head and the lathe bed. The lathe bed is only connected to the lower structure (wiring cabinet) by a couple of bolts anyway.