Postal meter marking

[GBEAR1]

Hi. What is the meaning of a U stamped on a National postal meter receiver below the ser no? Is it rare and does it increase the value or desirability of the rifle? Thanks

[jim c 351]

Receiver was made by Union Switch and Signal. Don't know production amounts
Jim C

[Johnny P]

Union Switch & Signal had a contract to supply National Postal Meter with 90,000 M1 Carbine receivers, and delivered 50,052 between August of 1943 and March of 1944.
US&S also supplied NPM with 64,602 slides and 54,593 bolts during the same time period.
US&S was also to supply Rock-Ola with receivers, but none were delivered. Another 28,949 receivers were delivered to Quality Hardware & Machine.