
You're correct on both counts; yes, the modules are interchangeable, and the Ladies' display is smaller. The Dress module uses the same display as the P4 models. BUT, having said that [and seemingly clarified the situation], I must also add that there IS a THIRD, mid-size-range display that I have seen on both Dress and a P4 model [Executive]. Seen individually, they are almost indistinguishable, but, held side-by-side, the difference in height is very noticible [a lot like the odd, round-face Executive cases that were hiding in the general population of watches...unnoticible until you held one in the same hand with a standard one]. Check your watches, gentlemen; I'm sure someone else out there has these variations too... And then there is the extremely rare dot-segment display found on a tiny percentage of the [earliest] Ladies' modules... but I sense I am drifting a bit far afield here...
As for the eBay auctions, my guess is the subtle difference is lost on most sellers, especially the ones that do not specialize in LEDs; there's a module in the watch, it lights up...so it's good to go.