We've an earlier thread on the Astronaut, assuming it's the same model -
https://www.vintage-radio.net/forum/...d.php?t=160118 -
identifying the cartridge as the BSR X5M, a mono cartridge but fully compatible with stereo records - it seems very unlikely that anyone will have fitted an older non-compatible type. With a mono record player I'm not clear how a stereo cartridge might be wired to avoid the possibility of cancellation effects - clearly using just one channel of its output would alter the listening experience in an undesirable way (exactly like silencing one speaker of a stereo system).
Others may have a better idea than I do of how well X5Ms tend to be performing now they're a few decades old.
Paul