To be pedantic a vehicle is classified as 'Historic' and MOT and VED exempt at a rolling 40 years of age from Date of First Manufacture, not registration.
I've had a couple of 70's motorbikes reclassified 'Historic' on account of them being in the dealer's showroom for a year or so before they were sold and therefore had been registered with a V5c which reflected the date they were sold.
It was simply a case of proving the date of manufacture and getting the class changed at my local post office (plus a refund of the remaining VED that I'd already paid).
I'm not sure whether the date at which a m/h is classed as 'manufactured' would be that of the original base vehicle (I suspect it is) or the converter's date. After all, if you convert an ambulance/Transit van to a m/h the date of manufacture isn't changed and you don't get a later reg'n number. From what I've read on here it can take 6 months or more before a base vehicle becomes a m/h, is sold by the dealer and is registered with a V5c.