MOT exemption is based on date of manufacture...not date of first registration...So first look in your V5C and see what is entered there...Also, go onto the DVLA vehicle enquiry site, put in your registration number and make and you will see what information they have regarding this on their database...
If it is 'declared manufactured 1967' you will need to get in touch with the VMCC library...They have a lot of the production/dispatch records for the M20....
If you provide your frame number (and I think they want a frame 'rubbing' and photos of the bike as well) they will let you have a copy of the relevant record which is accepted by the DVLA for the purposes of changing the declared date of manufacture....They charge for this service...
DVLA.. DO NOT.. accept a 'normal' dating certificate for this purpose...Though bizarrely they do accept that as proof of age if you are applying for a first registration on a 'barn find' etc...
But then, it is the DVLA which explains a lot....Ian
email (option): ian@wright52.plus.com
VMCC dating letter no good (tried that) you need a certificate from BSA owners club for them to believe you....then send your photocopied paperwork to DVLA, customer complaint resolution team,C1 west, Longview rd, Morriston, Swansea SA6 7JL
P.S you will need frame rubbings plus eng. no. rubbings plus close up photos of numbers