Dating a carbine by its serial number alone is difficult and not always accurate given the circumstances inherent with manufacturing and the logistics situation during the time the carbines were manufactured. Some people estimate a carbine's date of manufacture by adding the quantities manufactured month to month to the assigned serial number blocks. This is not an accurate means of estimating the date the carbine was finally assembled or when it passed its final inspection.

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    Serial number blocks were assigned by the Ordnance Department on an as needed basis, generally in response to the awarding of a contract to a given manufacturer. Think of each serial number block as a separate contract. Contracts had a completion date but there was no requirement for completing one contract before starting another. There was no requirement as to using the serial numbers in order.

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    A few manufacturers used some serial number blocks out of order. Some of these blocks were split up and used before or after other blocks. Some manufacturers used more than one block in the same time frame.

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    A few of the manufacturers used subcontractors to manufacture their receivers. One manufacturer separated out smaller blocks of serial numbers for use by subcontractors who would use these serial numbers concurrent to receivers being made by the prime contractor and/or other subcontractors assigned lower or higher serial number blocks.

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    The serial number was placed on the carbine receiver during the manufacturing process of the receiver, not when the carbine was finally assembled. The decision for which receivers would be used for the U.S. Carbine Models M1 or M1A1, and some of the M2's, were made well after the receiver was completed as all of these models used the same receiver.