Maybe not with the intention of housing a Cal 15 as that wasn't around at the time, but perhaps for another model that never saw the light of day. And then those cases were rediscovered when the C15 was being introduced and used for the earliest Monacos.

But we can be absolutely sure that a 16xxxx serial would not have been contemporary with Cal 15 production - we know launch Monacos in 1969 have 157 serials and we have a document that estimates total production in 1970 and, if I remember right, 1971 too to the effect that the serials have increased significantly beyond that by the time of the Cal 15 being introduced.

So there remains something of a mystery why these older cases are being used by the factory for a newly introduced model.