New Old Stock "Post Top Streetlamp Fitter"  aluminum HALF  casting  , #801. as shown; Could be modified for

USE AS A BIRDHOUSE, VERY UNIQUE, It would LAST FOREVER!

this is an unmachined foundry aluminum casting, very unique,

and you could join 2 together, or mount the half on a surface, wall, etc.

specs  8" wide.  5  3/8" high,, 2  3/8" is the zenith of the arc  weight 1#  13 oz

Reminds me of an Ancient Greek or Roman column top;, Doric