Cheers Alex. It's been painful letting some of my most prized items go, but needs must. Moving to a smaller place involves sacrifices...
I think the diopter on the mod 60 was a rather heavier steel framed sight whereas the mod 65 had the lighter alloy model also used on the 75. It may just be that the examples of the 65/66 that I've seen had later pattern sights fitted, or that there was a transitional period between the two styles. In any event, they are interchangeable.* I doubt if Diana still sell them but they still come up on eGun very frequently - and it's easy to have them sent over.
* Actually I may be wrong about this: See HERE.