An update; I received my 2nd touch screen from our Man in Hong Kong yesterday. Not only was it the correct size this time, but it was already mounted in the frame, so I didn't have to dork around cutting the old screen out and gluing the new one in. I have no idea why some of us have received different products from this supplier, but I am glad to report that "we do the happy and successful transction".
All hail Frank S, pretbek, and GS Addict for showing the rest of us the way.
More info for those that want to try the Zumo 550 screen-ectomy:
I received both the incorrect screen (too small, connector in the wrong location) and the correct screen (79.3mm x 64.77mm aka 3.125" x 2.550") from "Yang Lin Sen" at "Espeed Logistics Int'l Ltd." in HK. He and I emailed back and forth from the "mingxing200808" link on eBay. I believe he is the same seller that GS Addict bought from.
When I told him that the first screen was too small, he replied after a few days, and I told him the correct size again. The second screen arrived in less than two weeks, a week sooner than he had predicted. He has not asked that I return the first screen, or asked me for more money.
Part number on my old touch screen = 402D 035-41 DA
Part number on the 4 pin connector = A68-D35BA402D 750006400722 0264
Part number on new touch screen = 402D 035-25 BE
Part number on new connector = A68-D35BA402D 710014410703 0 ?? 72 (the third digit was obscured by black ink.)
If you receive a replacement touch screen already mounted in its frame, it's a snap-fit onto the LCD screen. The trickiest part for me was to gently bend the black frame of the old touch screen a bit on all four sides so it would un-snap from the LCD screen.
When re-assembling the 550 case and the guts, the 4 pin connector on my touch screen came out of its socket twice as I was positioning the pieces together.
I didn't have to re-calibrate the screen; it worked correctly when I powered the unit back up, but I shut it down and re-calibrated anyway.
There were small scuffs and scratches on the metal frame of the replacement touch screen, it may have been "refurbished", but it seems to work okay. So far.
Again, my thanks to Frank S, pretbek, and GS Addict.
Ran Bush in Nor Cal