This is an equalizer bar or pivot bar is used on tandem axle trailers to attach the leaf springs on both of the axles together. This equalizer bar is hot dipped galvanized and has 9/16" inside diameter nylon bushings. It measures 7 3/4" center to center across on the bottom 2 holes and 2 7/8" tall. This is probably the most common equalizer bar on trailers with 3,500# axles. It is commonly referred to as EQ-E1. This equalizer bar is for use with 1 3/4" wide springs. Since this pivot bar is galvanized, it works great for boat trailers.
The purpose of the equalizer bar is to help evenly distribute the weight on the trailer between the two axles. As the suspension articulates the equalizer will pivot in one direction or the other to try to keep the load distributed between the two axles instead of letting all the weight transfer to just one axle.
The 9/16 Inner Diameter bushing that comes pre-installed is suitable for most land trailers. If the purpose of this equalizer bar is for a boat trailer, you will most likely need to replace that bushing with a bushing with a 1/2 inch inner diameter. The appropriate one for this Equalizer bar measures 11/16 Outer Diameter, available here Trailer Leaf Spring Bushing 1/2 inch ID, 11/16 inch OD