This goes back to the beginning of the auto 5 production in 1903. John Cockerill was a British Industrialist whose company supplied high quality steel to Fabrique Nationale (FN) which was the maker of the A-5.

The Cockerill name was first used on Browning barrels stamped "cockerill steel". The steel was a very strong quality that met the proofing requirements of guns that were shooting smokeless powder. Eventually the Cockerill name was dropped from the barrels and "special steel" was used instead.

The FN guns that were not sold to Browning in the US, uses the barrel stamp of "acier special" which means special steel in the French language.