Looks to me like the movement has been replaced? It is a very old watch and a lot can happen over time, but surely more likely that the movement changed rather than the case back? Are the case backs even interchangeable between -1 and -3?
I am not good enough with early dials, but I suspect this is a 2915-3 with a replaced movement. The later tach adds weight to my theory, though of course these can be changed as easily as a strap.
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