What it is
The Zurn Wilkins 350 is a large double check valve assembly in 2½" through 10" (12" on the flanged version) in the same three bodies as the 375: 350 flanged ductile iron, 350A grooved, 350AST stainless. The 350DA / 350ADA / 350ASTDA add a metered bypass for fire service detection. NRS, OS&Y, butterfly, and post-indicator shutoffs.
With no relief valve, the body sits only 3½" below the pipe at 2½" and 8½" at 8". Everything above is shutoff: OS&Y stems at 11½" to 24¾" above centerline.
What matters for the cover
Read the letter. Flanged, grooved, and stainless bodies differ by up to 8" in length at 8". The 120 × 50 cover clears all three at that size; at 4" and 6", knowing which you have decides whether the 85 × 40 or the 100 × 50 sits right.
The base can close. No relief discharge, so the cover base closes fully to the pad. Do that: it holds ground heat and is the reason a DC under a fitted cover rides out weather an RP would not.
350DA bypass. The detector versions hang the bypass meter and check along one side, roughly 12" to 15" of added width on the stainless version. Photos first on the DA, especially on 6" and up.
Fire lines cannot be drained. A 350DA on fire service stays charged all winter. In cold climates that is a heat question, and the fire authority may require a heated enclosure; see the fire protection page before quoting a cover alone.
Stocking
The 350DA is the fire-line assembly in Wilkins territory, so fire protection distributors drive the 4" and 6" cover volume. The plain 350 shows up on irrigation mains and low-hazard commercial services through plumbing houses. It shares covers with the 375 size for size, which keeps the shelf simple.