Serial Number Placement:
All M1911s have only one serial number location and it is on the receiver. In 1937, following a long period with no pistol procurements, the military pistol production restarted (at serial #710001) and Colts began applying the serial number to the slide as well as the receiver.  This numbering of the slide and receiver was their normal commercial practice at the time.  Colts continued the double numbering until about serial #1140000 when it was discontinued as unnecessary. 

On all M1911A1s (1924 to 1945) produced by all manufacturers the serial number is located on the receiver between the stock and slide stop hole on the right hand side.

Colt Serial Number Styles:
First Roman to about Serial Number 4500, then Arial/Block to end of M1911 production. When production started again in 1924 It then went Roman again at 700001 to 1945 at about Serial Number 2270000 when it went back to Arial/Block
 

Note: Due to military maintenance practices as well as barracks cleaning mix-ups, 1911A1 numbered slides were often mixed up and a number of 1911A1s in this serial number range will be found with numbers that are off by only a few numbers.  Many others will simply have mixed up slides and receivers.  A non-matching slide will reduce the value of a pistol to a collector considerably.  Yet pistols just beyond where the slides were numbered may bring considerably more money than a numbered pistol when the slide just a few numbers off. Yet there is no way to tell if the un-numbered slide matches either.

Commercial Note:  
In part of the year 1920 some Commercial Government Model pistols  had a serial number on the slide underside in front of  the disconnector well as well as on the receiver. This practice only lasted a few months and the slide serial number was moved to underneath the firing pin stop plate.  Some military pistols brought back by returning WWI vets were apparently sent to Colts for refinishing in that small time slot, and while refinishing, Colts applied their military serial number underneath the slide as was their standard practice for those months. Reference Charles Clawsons “Colt .45 Service pistols”.