1924 would put your gun in the first year's production of the A1.
Since this is a commercial gun, the letters could be a company or police dept control number.
Unfortunately, old guns very often had mysterious numbers and stamps on them that cannot be ID'd at this late date.
At the time, SOMEBODY had reason to go to the trouble to stamp the guns, but usually fail to record the REASON, or WHO did so.
So, unless there are more guns out there with simular marks, and some collector managed to find out what they were for, I doubt you will ever know what they were for.