Another possibility is that it is a 1970 Thunderbolt engine and someone has modified the Y suffix to read XD.
That would make a lot of sense and remove some of the possible confusion the number would have otherwise generated.
If this is the engine then I doubt the factory would have been involved in the stamping... the 'Y' engines had been in production for many months by December (about 6 months).
As Larry has suggested, the XD could be taken as Dec 1969 for a 1970 engine and at least then you would be buying the correct parts for it.
All 1970 model A65 engines, apart from the 'Y' models, had five digit serial numbers.
Last edited by Kevin (NZ).; 03/12/13 11:33 am.