[serf 'rett]

Nope, I don't think so.

Some type of metal slag or something embedded when the brick was made.

The trough below the metal is too uniform for this to be bullet impact. Get a magnifying glass or jeweler loupe and see if there are small pieces of lead or gilding bronze in the trough. I suspect you'll not find any there.

The angle of a projectile would be too shallow to cut the trough and then bury into the top side of the brick. The shallow angle required to cut the trough, would almost certainly cause a high velocity projectile to skip off the hard surface.

The projection of flakes of brick over the metal tends to rule out an anchor or fastener. Drilling holes for leadheads, redheads, potmetal zinc, etc. anchors – the mouth of the hole is often enlarged. Try to drill at too shallow an angle and the hard brick will pop apart above the bit.