The Xbox 360 Winchester motherboard was introduced along with the Xbox 360 E Slim during the Xbox One introduction in 2013. The board derives heavily from the previous Corona configuration with a few small hardware differences and security fixes.
The Winchester board is nearly identical to the previous Corona models with a few small differences. Most notably, this console is incompatible with RGH/RGH2.
The Winchester board was exclusively used in the Xbox 360 E Slim model. Early Xbox 360 E Slim models used a final revision Corona board slightly modified in order to liquidate their stock.
Case differences aside, the biggest differences with the board include the XCPU missing the IHS, the digital audio out port being removed and a single USB port missing.
There was never a full Xbox 360 E Slim or Winchester based XDK available for developers. There are a few rare stress boards that were most likely used for evaluating the motherboard before it was released at retail. The Winchester Stress kits look identical to retail Xbox 360 E Slims with stress wires hanging from the side and an internal asset sticker on the side.
There was never a sidecar designed for the Winchester board although the slim sidecar daughterboard is present. The Winchester Stress Kit is also locked to 512MB of RAM usage. They may have 1GB of RAM but it isn't usable. These Stress Kits all report as "Test Kits" although it is unknown if adding a Sidecar will convert it to a full Development Kit.
The lowest recovery that can run on these Stress Kits is 21256.