Thinking about this further, I think I know what's going on, because it's exactly what's going on in my own industry. I work in the semiconductor industry for a company that manufactures its own chips for inclusion in other products.
A small number of the chips we manufacture are also built out to fully working products with our own branding, which is how we satisfy initial demand for the category of products that we make. Whereas we make the chips, we do not do manufacturing or assembly on the other ingredients necessary to make that fully-working product. A third-party company integrates the chip into a final product of our exacting specification, including: colors, shape, branding, circuitry, etc. We provide the schematics to an OEM with some chips, and they create the remaining constituent parts with their network of suppliers and to assemble a working product.
The next batch of chips we manufacture after that initial run also go to that OEM, except this time they don't put our branding on it. We sell our assembled design, unbranded, to a variety of partners that apply their own branding and tweak the parameters for final sale to the consumer at a markup over the cost we sold to them.
The next batch of chips after the partner phase don't even get assembled by that OEM, necessarily. We just sell the chips to our partners, and they integrate them with unique product designs that--except for the chip--could vary wildly from the design we initially made under OEM contract when the product was introduced. We have a share in the revenue. Hell, those partners of ours could even go back to that same OEM if they choose to, but there are many like that OEM.
Does anyone see where I'm going with this?
I think PTS Resources is that OEM for the watch industry. I think they offer an absolutely enormous number of case, dial and strap designs that even the Big Three draw on to satisfy their desire to have a comprehensive and interesting product line. I think companies like Shanghai, SeaGull and, to a lesser extent, Beijing all occasionally buy/license case/dial designs from PTS Resources, or use PTS' manufacturing/design capacity to augment their in-house manufacturing volumes.
And whereas my industry's chips cannot be used in a product from anyone else but us and our network of partners, I think Seagull/SWAF/BJWAF all sell their movements to PTS Resources in bulk so PTS can re-sell those movements to other parties like Ingersoll. Companies like Ingersoll can rely on PTS Resources' design/manufacturing capacity in the same way Shanghai can once that transaction is completed.
This is the only hypothesis I have that would explain why a Tier 1 movement maker would have the same designs as Chinese brandholders or western brandholders, or why those brandholders would have movements from any one of the Tier 1 movement makers out of China. PTS Resources is at the center of it all.
//EDIT: And I think SeaGull is so large that it performs the same function as PTS Resources, except they naturally utilize SeaGull movements alone. This would explain why the M172S design can be had from a large number of brandholders. For example: Langengrad, Maurice Blum and AWC all utilize SeaGull as an OEM. Many designs for these companies have all been found with a SeaGull brand stamp on them at one time or another.
