Serial number 618603 is a Model 1911 number. Ordnance toyed with the idea of converting the 1911 receivers to 1911A1 configuration, but decided against it. Occasionally a rebuild from Anniston Army Depot will show up with the finger cutouts, but it was not officially done.
Along the same lines, the WWII Remington 03-A4 Sniper rifle receivers were still marked 03-A3, supposedly so the receiver could also be used on a standard 03-A3 rifle. A few of the sniper rifles have shown up with the 3 in A3 obliterated and a 4 stamped in it's place. I always figured that someone at a rebuild facility took it upon themselves to correct the marking, and not done officially.
When the so called "Transition" pistol came out in 1924 it was to be an "improved" Model 1911. In 1926, Ordnance decided that the pistol would be a 1911A1, and that any pistol under serial number 700,000 would remain a 1911, and any pistol above serial number 700,000 would be a 1911A1. Model 1911 production ended at serial number 629,500 in 1919.