Burch, the FW means Featherweight, the S means it is now from the L.C. Smith Gun Company a subsidiary of Marlin Arms.
I think you put a 1 in front of the serial number, maybe it should be 43xxx which would be from 1949. Serial number starting with 143, would be from 1935 and would not have the prefix FW S.
The marks you see are proof marks. Having two ivory beads would be unusual, most just had a brass front bead. The barrel length means it was cut, as both Hunter Arms and Marlin did not make any fractional barrel lengths mostly even numbers, with a few exceptions of 27".
I would stay with loads under 9,000 psi and around 1200 fps. I would not use any magnum type loads.
