I too am able to tune 4905 khz in the AM from Las Vegas, NV. The signal is very strong.
Perhaps a dumb question but ... how do you know whether to tune USB or LSB for these types of data signals?
For example, it 'sounds' right at 4905 USB and LSB. I do not hear a difference.
This signal will sound approximately the same in USB and LSB when tuned to 4905 kHz because 4905 kHz is the center frequency, not the tuned frequency.
The center of the FSK is 4905 kHz. However the FSK is 850 Hz shift. So each leg of the FSK, the space and the mark, are 425 Hz below and above the center frequency of 4905 kHz. So when tuned to 4905 kHz in USB you are hearing the Mark (on 4905.425 kHz), and when in LSB you are hearing the Space (on 4904.575 kHz). And heard alone the space and mark sound approximately the same to the ear.
So to tune this signal you must decide if you want to tune it in USB or in LSB. Most utilities and ham data are tuned in USB, however RTTY traditionally is tuned in LSB. Some RTTY, depending on the source, is in USB, other sources will be in LSB. It is acceptable to tune RTTY either way, USB or LSB, however you might have to click the "invert" button on your demodulator.
This is a military FSK encrypted signal (RTTY is FSK, this signal is specifically called "STANAG 4481 FSK"), and there is no one tuning offset that is correct in the civilian world to monitor it. There absolutely are standard tuning offsets for various civilian RTTY shifts, but this really does not fit those. In this case you might tune to 4903 kHz USB, that would put the space at 1575 Hz audio and the mark at 2425 Hz audio. If you are using a 2.4 kHz filter that might put mark outside or on the edge of your passband, and you might have to adjust this, maybe tune to 4903.5 kHz USB, putting the space at 1075 Hz audio and the mark at 1925 Hz audio.
T!
(edit) I see that while I was typing this Josh was responding, and said much the same.
(edit 2) I forgot to mention the source. This is probably US Navy, NPG Dixon, CA.