Are the sides tapered? If so, it's one of a pair of mounts (the front one, I believe) for an old fashioned Unertl or Lyman externally adjusted telescopic rifle sight. If not, it would appear to be some sort of shim or spacer. Or a left handed whatsis?
Froggie
.143" and I think you might be on to something Green Frog. It may be a sight mount base because there is a slight bevel on one side that would fit onto a barrel. Maybe a scattergun...