I think you are right to be concerned for this watches originality.
I have not seen a -66 with a 240x xxxx serial, only as you say, -65.
The case back on this watch is clearly a -66HF.
The caseband shows clear facet junctions on at least two of the lugs. The case also looks more like a CB case in shape. ( I am still working on comparing those two but I am not with watches at the moment).
I am not sure the hands on this watch have spent all their lives together.
The dial and bezel look very nice indeed.
Case shows signs of salesman type polish and it looks like a new crystal.
I have had words with an older watchmaker who worked on speedmasters in the 1970's. He said he would often mix up cases/ case backs as he serviced speedmasters in batches. I must say I didn't like his attitude, but I was buying some parts off him.
However this watch has a non matching case back and case band, as well as a movement number in a range not yet confirmed as possible for any -66, be it HF or CB.
If I was offered this watch, I would try to buy it because the dial and bezel are so good, however I would not value it as an original watch.
That said, the dial and the bezel are really good....
Click to expand...