What it is
The Febco 850 (850U union ends, LF850 lead-free) is a bronze in-line double check valve assembly in ½" through 2". Two independent checks under one access cover, four test cocks, ball-valve shutoffs. It protects against low-hazard cross-connections, which in practice means irrigation and some commercial services in jurisdictions that allow a DC there.
What matters for the cover
Low profile. With no relief valve the body sits 1¼" to 2¼" below the pipe centerline and about 3"–4¼" above it. A ¾" 850 is roughly the size of a loaf of bread. The 20 × 20 closed-top cover fits it with room; the 24 × 24 fits the 1".
The base can close. A double check does not discharge, so the cover base can close all the way to the pad. In wind that is the difference between a cover that holds and one that lifts.
Length matches the 860. 10¾" at ¾" through 17⅝" at 2", so the 30 × 24 is the closest fit at 1¼"–2" and a union-end 850U may take the 36 × 24. If your counter already knows the Febco 860 covers, the 850 uses the same ones.
Height with the riser. The valve is short, so on a tall riser the riser is most of what you are covering. Measure pad to top of the test cocks.
Stocking
In jurisdictions that allow a DC on irrigation, the 850 outsells the 860 on residential systems and its covers move with it: ¾" and 1" in case quantities for irrigation supply. Where the utility requires an RP, the 860 takes over. Either way the same two small covers serve both, which keeps the bin count down. The DCV covers page covers the category.