A lovely picture! If the reg is OD 7977 (it's almost certainly that) it is J4412. The first owner, according to the Factory records was a resident of Beaworthy, Devon, but alas we have no owner info ever since then. The car was originally red and had red trim. (This pic is already on our database record for the car.)
Gotta add my two p worth. I do not think the numbers are 7973 but rather 7977. Looking closely the bit that makes the 4th digit look like a 3 is actually a fastener from the taillight.
The letters could even be BD.
Tommm on the sunny hot breezy sandy Lake Erie shore