This old driving bit is made of steel, there is some surface oxidation which could be burnished off.
It has a straight bar mouth which can turn.
The straight cheeks have two rein slots for the attachment of the curb rein. One cheek is stamped "Porter", the maker or retailer, and the other "Mayfair", a fashionable area of London. Unfortunately I cannot find any records of Porter as a saddler or loriner.
There is an curved shackle or bottom bar.
Mouthpiece 14cm (5½")
Cheeks 22.4cm (8¾")
Weight 751g (1lb 10½oz)
Ref : bit482