I have had a couple of those. I am not one hundred percent on this answer. But it was said to me that it was service dial. Not sure if correct or not this info.
321 Service dial... pre 78 is my guess. My theory is that they ordered small runs and each run was slightly different based on the person ordering and the screens for the current dial.
I have one of these. They are legit. Note the step, that’s what makes it weird. It was apparently a very short lived 321 service dial from the early 1970s and is mentioned but not pictured in MWO. The step on mine is shallow and this pic doesn’t show it well but there is a noticeable step.
They are actually 321 movement dials (or at least mine is) so really a kind of 145.012-68 pre-post transitional. A couple more shots showing the step and non AML more clearly.
Looks like they made them for both movements then unless it was bodged to fit. That’s strange as there are no other 861 service dials, they tended to fit later regular production. Not sure why someone would choose to fit a long index step dial to an 861 when servicing it, or why Omega would produce them since post ‘68 the dials went over to short indices. You don't see many of these either way.