Ny net / single letter beacons (D/P/S/C/A/F/K/M) :

The soviet/russian navy has an interesting system to let their ships abroad check for a suitable frequency: within a spectrum of less than 1 kHz, a maximum of 8 different stations can be heard, and the radio officer is able to check within very short time, which stations are active, and on what frequency range communication might be best. There are 9 frequencies to look for.

 

this recording was made with an online receiver in Finland, you can hear D,P,S,A here you can see it (both: rb, 2021)

here an online recording from Japan, listen to M,K,F,A see it in the audio spectrum (both: rb, 2021)

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D	Sevastopol	45°N/033°E	3594.7	4557.7	5153.7	7508.7	8494.7	10871.7	13527.7	16331.7	20047.7
P	Kaliningrad	55°N/020°E	3594.8	4557.8	5153.8	7508.8	8494.8	10871.8	13527.8	16331.8	20047.8
S	Severomorsk	69°N/033°E	3594.9	4557.9	5153.9	7508.9	8494.9	10871.9	13527.9	16331.9	20047.9
C	Moskva	55°N/037°E	3595.0	4558.0	5154.0	7509.0	8495.0	10872.0	13528.0	16332.0	20048.0
A	Astrakhan	46°N/048°E	3595.1	4558.1	5154.1	7509.1	8495.1	10872.1	13528.1	16332.1	20048.1
F	Vladivostok	43°N/132°E	3595.2	4558.2	5154.2	7509.2	8495.2	10872.2	13528.2	16332.2	20048.2
K	Petropavlovsk	53°N/158°E	3595.3	4558.3	5154.3	7509.3	8495.3	10872.3	13528.3	16332.3	20048.3
M	Magadan	59°N/150°E	3595.4	4558.4	5454.4	7509.4	8495.4	10872.4	13528.4	16332.4	20048.4

The above sheet shows the telegraph letter used (morse code, A1A), the naval port with the approximate coordinates, and the carrier frequencies in kHz. Not all stations operate on all frequencies simultaneously.