Since approximately 1927, Spanish law has required that every firearm manufactured in Spain be submitted to the Eibar Proof House for testing prior to being released for sale.  The testing includes a detailed visual inspection as well as the firing of one or more overpressure or "proof" cartridges, followed by further visual inspection.  If the firearm passed both the visual inspection and displayed no ill effects from the proof firing, it was stamped with a variety of proofmarks.  In addition to the proofmark, it was stamped with a letter code indicating the year that the arm passed proof.  The year code may appear on the frame above the trigger or under the grip panels.

The codes began with A in 1927, and after reaching Z in 1954, continued with A1 in 1955.  Likewise, the year 1981 was represented with the code A2.  The table below indicates the various year codes.