As stated on the page on assignments of FM station call letters, a new call-letter plan was adopted in 1941 that was intended to be systematic; first a K or W, then two digits representing the frequency, then one or two letters representing the location. Frequencies at the time were in the range of 43 to 49 MHz, and the two-digit frequency code was obtained by dropping the "4" and the decimal point; thus W81SP was on 48.1 MHz in Springfield, Massachusetts. This 1941 systematic call letter scheme lasted only until November 1, 1943, at which time the call letters were normalized to a similar scheme to the AM broadcast calls, except for some experimental-type calls.
The following table shows the changes made at that time (details on the individual stations can be obtained by linking to the individual stations' chronology pages):
| Old call letters | New call letters |
|---|---|
| K45LA | |
| K47SL | |
| K49KC | |
| W31NY | |
| W39B | WMTW |
| W39NY | |
| W41MM | |
| W43B | WGTR |
| W45BR | |
| W45CM | |
| W45D | |
| W45RG | |
| W45V | |
| W47A | |
| W47C | |
| W47NV | |
| W47NY | |
| W47P | |
| W47PH | |
| W49B | |
| W49BN | |
| W49D | |
| W49FW | |
| W49PH | |
| W51C | |
| W51NY | |
| W51R | |
| W53H | |
| W53PH | WFIL-FM |
| W55M | |
| W55NY | |
| W57A | |
| W57PH | |
| W59C | |
| W59NY | WQXQ |
| W63C | |
| W63NY | |
| W63SY | |
| W65H | WDRC-FM |
| W67B | |
| W67C | |
| W67NY | WABC-FM |
| W69PH | WCAU-FM |
| W71NY | WOR-FM |
| W71RF | |
| W71SB | |
| W73PH | WPEN-FM |
| W75C | |
| W75NY | WABF |
| W75P | |
| W81SP | |
| W85A |