It's a common mag that is also used on Morris, MG's, Rovers and others. It's likely it was for your Dad's Anzani-engined AC - it would work OK, looks to have the right coupling, so only rotation would need checking. If it's been lying around in the garage, especially if damp, it may well need a service, even a rebuild. A number of companies offer that specialised service. Pity no-one with a 4-cylinder has been in touch....but it will sell alright.