What it is
The Watts 774 is a double check valve assembly in 2½" through 12" with a 300-series stainless body, cam-check modules under a single grooved-coupling top cover, and no relief valve. The 774DCDA is the same body with a metered bypass for fire service, so a fire sprinkler system's water use can be detected.
Because there is no relief valve, the body sits only 3½" to 8" below the pipe centerline. The height is all shutoff: open OS&Y stems reach 16½" above centerline at 2½" and 45¾" at 10".
What matters for the cover
Length. The 774 runs 37" at 2½" and 40" at 4", noticeably longer than the 957 or Colt families at the same size. The 70 × 40 cover is the price-list match at 3" and the 85 × 40 at 4"; both clear the standard gate build.
No drainage worry. A double check does not discharge, so the base of the cover can close fully to the pad. That is a real advantage in wind: no gap needed anywhere.
The DCDA bypass. The detector version hangs a ⅝" × ¾" meter and bypass line along one side, adding several inches of width. The stock cover usually accommodates it on 2½" through 4"; on 6" and up send a photo with the bypass side visible so we can confirm or cut a custom pattern.
Top clearance. The single top cover needs room above it for service, which is a plumber's problem rather than a cover problem, but it is why the cover should lift straight off rather than wrap tight over the coupling.
Stocking
The 774DCDA is the fire-line assembly fire protection distributors see most in Watts-spec territory, and fire lines cannot be drained for winter, so every one that is outdoors needs protection. Stock the 85 × 40 and 100 × 50 for 4" and 6" fire services. Domestic 774s on plumbing wholesale accounts run smaller.
For the DCDA category as a whole, including bypass clearance and the fire marshal's role, see the DCDA covers page.
Ordering
Name the size, the shutoff type, and whether it is the DCDA. With gates, the 70 × 40 covers 2½" and 3", the 85 × 40 the 4", the 100 × 50 the 6". The bypass-side photo settles the rest.