Acier special means Special steel in French, and barrels marked this way are only on FN sales guns, not Browning Arms Co. guns. Fabrique Nationale (FN) was the manufacturer of… Auto 5's, for the Browning Arms Co. FN also produced the guns for sale outside the US and were not imported to be sold by the Browning Co..
Providing there is no other letters as part of the serial number, this gun was made in 1928. It should be an FN Auto-5, not a Browning Arms Company A5. The gun will likely have the Fabrique Nationale de Herstal address along the ridge of the receiver, but no receiver engraving. The "c12" simply means 12ga. Cartridge..
The FN version is mechanically the same as the Browning version, but markings and finish may be different..
Although the Blue Book of gun values doesn't differentiate these guns from the ornate Brownings, prices are lower because of the engraving. If your gun has an engraved receiver, then it's possible that the barrel was fitted, and not original to the gun..
Value is too difficult to be accurate without seeing the gun. Condition which includes the amount of original finish and original configuration will determine value. Auto 5's with recoil pads (other than Mag 12's) poly-chokes and compensators will reduce value..
Recommend taking it to a gun shop for appraisal, but keep in mind that few appraisers know the information and history of the FN guns. ( Full Answer )