I don't think the XD is anything to do with the later coding system introduced during the 1969 season.

The Month/Year codes were always as a prefix.. from what we have been seeing so far it is only the 'Y' and 'X' bikes that had a suffix code.

I am pretty sure this is going to be a 1967 model engine.
It will be an interesting photo. As I see it at the moment all three portions of the S/N are contradictory.
A photo should be able to date the engine in itself, without reference to any markings.

This is going to be an interesting one.
The serial number itself would normally be considered too low in the model year to have earned a suffix.

Why, Y, Dash Y..