hi Terry ;
that may be a swindon or wiltshire bike with the AM suffix .
i have a 1963 puch cheetah which is 993BAM and i live near swindon and the local registrations used to use AM MR MW and i think WV and maybe others . .
my 65 triumph T10 scooter is GMR311D again from swindon .
i have a very late sports arrow in bits at present that came from a local fella and was a chippenham wilts bike but dont have the number to hand at present as its tucked away behind some junk .
at some point the county boundaries were moved around in many places and so it might be your bike was registered in wiltshire but the county name is now different .
it happened in oxfordshire near me where some of the area used to be berkshire .
i have a sept 58 leader which is 669DJO and this is its original oxfordshire registration number and also a 1922 carfield which is BW7022 and its original oxfordshire reg number .
hope you get some more history sorted out on your ariel .