I got what I thought was a very informative reply back from Dorman in response to my inquiry. Obviously they're trying to sell their stuff, but it did give me some reassurance...
Our wheel cylinders are all manufactured to meet or exceed "OE" wheel cylinders. They are engineered and manufactured to conform to the following SAE standards:
J431 for iron castings (hardness, tensile strength, etc.)
J101 specifically for wheel cylinders (apply and release, ozone resistance, static leakage, applied leakage, storage corrosion)
J1601/1604 specifically for rubber components (we use EPDM compound rubber which is compatible with glycol-based fluid, resistant to water at elevated temperatures, resistant to heat and ozone, oven aged for hardness and shape conformity, and many other attributes).
The Motorcraft cylinders are not original Ford "OE" cylinders, but aftermarket replacements like ours, Raybestos, Wagner, Bendix, etc.
The correct cylinder for the Contour 4 cyl (8" brks) is W37997. The 9" brks were used for the 6 cyl, which is our W610056. Following is the correct "OE" part number interchange:
4 cyl 8" brks: Ford F5RZ 2261-B (our W37997); Motorcraft BRWC33
6 cyl 9" brks: Ford F5RZ 2261-A (our W610056); Motorcraft BRWC43