It is a viewer's (inspector's) mark. It is read, B.S.A.T. for Birmingham Small Arms Trade. It is an interesting bayonet, in that it exhibits Drake pattern characteristics in the elbow, ricasso, and rear terminus of the blade flutes. The locking ring stop is in an odd position, which corresponds to the Sharps Drake Pattern bayonet in my collection. Augustine J. Drake was a Boston contractor, so may have sourced some of his bayonets from the Birmingham Trade. The Sharps Drake Pattern bayonet in my collection is unmarked. Drake also contracted bayonets for conversions of the M1841 Mississippi Rifle. The only way to make a good identification is through precise dimensions. Need the blade length (point to front edge of socket to nearest 1/16 in.); socket length, and socket diameter (ID @ front, in thousandths. Take several measurements and average them). A picture of the whole bayonet would also be helpful. It's a very scarce Civil War bayonet.
took the best pics i can with my small cam really not shure how the blade is measured? from tip to elbow or to? measurment s socket 3" rear hole i get about .820ish front hole about .795 ish front site about .282 wide about .290 deep in the nub about 20 1/4 total overall
Based on the dimensions, your bayonet is likely a Spanish M1857 socket bayonet. It is too short and muzzle ring too small for the Drake Pattern bayonets used with either the Sharps or Mississippi rifles. The Spanish M1857 is the "other" bayonet type known to exhibit the Drake Pattern features. Reilly's book on American socket bayonets indicates that some of the Spanish bayonets are known to have been manufactured in England and Belgium, so the Birmingham mark on yours fits. Spanish M1857 bayonets are scarce, but not as desirable in the USA, as the Drake variants used with American Civil War arms.