This is an old military Portsmouth or Elbow horse bit
It is made of steel which is dark in places but could be burnished back to a shine.
This design was used by the military because it was a "universal" bit - they often had reversible mouthpieces and the choice of rein slots made it suitable for many different horses.
Each cheek has two slots for the curb rein.
One cheek is stamped with a a large "L" possibly "Q" a broad arrow or ordnance mark and "15".
The other cheek is stamped "C α 3" The bit still has one original curb hook and a nickel replacement.

Dimensions : Mouthpiece 5¼" (13.2cm), Cheek length 7¼" (18.5cm)
Weight : 617g (1lb 5¾oz)