Guess that your best bet is a DVLA local office (before they close them all) and a face to face discussion. This may be stating the bloomin obvious but assuming that your machine(s) were bought retail in this country and not personal imports, any duty and vat due (vat not introduced into the UK until 1973) would have been paid at the point of import by the then Simplicity importer/distributor. Cannot remember who that was although the name Stemport Marketing keeps nagging – I know they imported Masport machines.
On edit – can’t remember what I had for breakfast but am right about Stemport!!!
https://www.gracesguide.co.uk/Stemport_Marketing_Co