This is a boat trailer leaf spring shackle link.  It is HOT DIPPED Galvanized steel.  It measures 2 1/2" from center hole to center hole and has an over all length of 4".  Each hole has a diameter of 1/2".  The shackle strap is made from 1/4" thick steel plate and then is hot dipped galvanized.  The galvanized finish helps to prevent rust in salt water environments.  These shackle plates are used to hang a double eye springs on a trailer.  They mount on the rear of the spring allowing the spring to lengthen as it absorbs bumps.  They are also used in tandem axle trailers to attach the front and rear springs to the middle equalizer bar.  Single axle trailers will use 4 spring shackles (2 on each side).  Tandem axle trailers will use 8 spring shackles (4 on each side).  This is the most common length spring shackle.

These plates can also be used as frame plates on the bottom of u-bolts, or as general brackets.