B-Line B202-1SS4 Eaton series strut fittings and accessories, 1.62" Height, 1.62" Length, 1.62" Width, .16lbs, Steel, Square washer, Hole size 11/16 in diameter | Bolt size 5/8 in, Stainless steel type 304

It measures 1.62 inches square, so the plate spans well past the open face of standard 1-5/8 inch strut and bears on both lips rather than pushing into the channel opening. That is the whole point of a square washer here: it spreads the clamp load of a 5/8 inch bolt across the channel edges instead of concentrating it at the hole.

Type 304 stainless. The material is steel with a 304 stainless specification, so it suits damp, washdown, and exterior runs where a plated or painted washer would bleed rust onto the strut. Pair it with 304 stainless bolts and channel nuts; mixing stainless hardware with plated hardware in a wet location is how you get galvanic corrosion at the joint.

It is Eaton B-Line part B202-1SS4, from the B-Line strut fittings and accessories line. The SS4 suffix is the 304 stainless version. Quote that exact number when you are matching an existing stainless strut system, since the same B202 plate is also built in other materials.

On strut channel, yes. A round washer tends to sit down inside the channel slot and dig into the lips under load. The square B202 plate, 1.62 inches on a side and about 1.62 thick overall in the stated envelope, sits flat across both channel lips so the 5/8 inch bolt clamps against a broad bearing surface. For flat plate or a drilled beam flange, a standard round washer is fine.