I have a cast iron bracket, single, vintage.  It measures about 7 1/2" x 5 1/2" x about 1 3/8 in width at the back where the screws fit in (it narrows slightly as it goes down on either side to 3/4" at the ends)   The original finish is spotted with surface rust.  It is in nice condition otherwise.

Bracket Cast Iron Single Salvaged Vintage