What it is
The Watts 957 is a reduced pressure zone assembly in 2½" through 10", built on a 304 stainless grooved-end housing with two check modules and a bottom-mounted relief valve. It shows up on commercial domestic service, on fire service as the 957RPDA, and increasingly as the replacement for a failed 909 because it fits in less pad.
Shutoffs are the tallest thing on it. With OS&Y gates the open stem tops out around 16 inches above the pipe centerline at 2½" and nearly 46 inches at 10". Ball-valve (QT) versions through 4" run much lower, and the BFG butterfly build is shorter end to end.
What matters for the cover
Length. The 957 is short for its class: 30¾" at 2½" against 41½" for a 909. Our 70 × 40 open-top cover is the price-list match at 3" and clears the 2½" and 3" assemblies with gates; at 4" the 85 × 40 is the match. Check the table below for the size you are covering.
Height. Open OS&Y stems set the height, and the relief valve adds 6½" to 11" below the pipe. The 40" and 50" cover heights are built around exactly this stack, but a tall riser under the assembly eats into that. If the pipe centerline is more than about 18" above the pad, send a photo before you quote.
Relief valve. The relief valve under the body has to drain. Set the cover so the base closes to the pad around the assembly but never seals under the relief port.
957N and 957Z. The N and Z patterns stand the body vertically on elbows. Those are N-shape cover installs, not open-top ones; pick the N-shape SKU for the pipe size.
Stocking
The 957 is the most common large RP on new commercial work in Watts-spec markets, and the 957RPDA is the fire-line version of it. Fire protection distributors stock the 4" and 6" covers; plumbing houses see 2½" through 4". Keep one each of the 70 × 40 and 85 × 40 and you cover most of the calls.
The 909 it replaces is still in the ground by the thousands. Keep the Watts 909 cover page handy for the older installs, and remember the two do not share a cover.