What it is
The Apollo RP4A (RPLF4A in lead-free; Apollo part numbers in the 4A-200 / 4ALF-200 series) is a bronze reduced pressure zone assembly in ½" through 2" with a bottom-centered modular relief valve, top-access check cartridges, four top-mounted test cocks, and ball-valve shutoffs. Union-end versions are available. It replaced the Conbraco RP40 (40-200 series), which is taller and still common on older installs.
Length runs 12⅝" at ¾" to 20⅛" at 2"; height 3½" to 5" above the pipe centerline and 3⅞" to 6" below to the bottom of the relief valve. Apollo lists an optional wye strainer shipped loose; installed, it adds several inches to the inlet side.
What matters for the cover
Standard small-RP covers. The 20 × 20 fits the ¾", the 24 × 24 the 1"; at 1¼" through 2" the 30 × 24 is the closest fit of the price-list group. A 2" with the strainer installed moves to the 36 × 24.
Relief valve straight down. 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.
The RP40 is a different envelope. The legacy Conbraco unit stands taller: 5⅛" to 7⅛" above centerline against the RP4A's 3½" to 5". A cover fitted tight to an RP4A is short on an RP40. The finder lists the RP40 separately for exactly this reason.
N and Z patterns on the RP4A stand the body on elbows and are N-shape installs; photos first.
Stocking
Apollo carries a lot of spec share in the Carolinas and the Southeast generally, and RP4A covers move in the usual small-RP ratio: mostly ¾" and 1", a trickle of 1½" and 2". Backflow testers on Apollo-heavy routes carry the 20 × 20 and 24 × 24 as truck stock. Pair with the DC4A, which shares the body and the covers.
Ordering
Name the size, say whether the strainer is installed, and say whether it is an RP4A or the older RP40; both change which of the 1"–2" covers fits. The 20 × 20 and 24 × 24 handle the ¾" and 1" without a question. Photos settle the N and Z patterns.