Let me float a hypothesis.
It was not unknown for Zenith to produce a limited run watch for a specific market and neither was it unknown for them to make use of items they found in a drawer somewhere, such as the late edition 135s.
Imagine they had found a drawer full of turtle cases after they had stopped making the turtle and rather than ask Singer to tool up again for the turtle dial, they just designed a new finish for what was a fairly ubiquitous dial layout in the early to mid 70s. This could be such a run and would also account for it not having a chronometre badged dial or movement. This dial will fit all of the 25xx PC movements so a 2552 in one and a 2562 in another is not out of the question.
As I said, this is pure speculation, without basis but with precedent. I will consider these to be the "bit-box turtles" from now on.