Oh piss, I'll answer since Howard ain't picking up the phone on this.
The Sunwatch has a solar panel cover without a notch in it for the center bar "digi" mentioned, and it obviously doesn't have a bar down the middle of the the metal case. Any Synchronar module could theoretically go into a Sunwatch case, but a Sunwatch module can't go in a regular Synchronar case because the bar would be in the way. A Sunwatch is in no way an inferior model, but it is(Howard correct me on this, I haven't memorized the history like you) about in the middle of the chronological order of Synchronar development. Somewhere around a Mark III in it's feature set. I personally adore the clean "barless" look of the Sunwatch. And some of them came in this really neat display bnox which is like an upright cylinder with a rotating outer portion that reveals the watch.
I'm pooped...I think I went on waaaaaay too long.
I think "digi" is right, the idea came up when some of the metal cases had a inferior bar down the middle, and it was decided that changing the module top was the lesser evil - I could be way off on that part.

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