I agree Eddie and that was the only worry I had (after the direction of rotation which is OK) since the Vincent ones are gear or chain driven, however there are a lot of bikes fitted with the magdyno model they have been selling for a long time and the latest Alton has a lot less rotational resistance and a auto cut out to stop discharge of the battery as per some alternators at rest it has the same silhouette as a Lucas dynamo
From their web site
Suitable for magdyno equipped motorcycles.
Suitable for Ariel Red Hunter (250-350-500cc singles), Ariel VB600,
BSA M20, M21, B31, B32, B33, B34, Gold Star DB34 & DBD34…
Norton model 50, Norton ES2 (up to 1958), Panther Model 100 & 120, Royal Enfield Bullet.
I only have a fleeting knowledge of some of them but I am certain the B31 has a slipper clutch (big round ended cruciform spring )