It's a universal (AC / DC) series motor, wired for DC operation through a bridge rectifier.
The motor reverses when its polarity is reversed, with the field winding polarity kept unchanged through the second bridge rectifier.
Dynamic braking is through the 'D' resistor, which is connected across the motor terminals when it is switched off.
The winch would function even without the two identical resistors, connected in series across the mains supply, at the junction of which one terminal of the motor is connected all the time.
The following is an analysis to determine their function.
The junction of the two resistors is a virtual neutral point to which one terminal of the motor is connected. The positive and negative terminals of the main bridge rectifier alternately get connected to the virtual neutral point when the motor is run in the forward and reverse directions. The purpose is not apparent.
Either resistor gets connected across one pair of diodes, when the motor is run in the forward direction, and across the other pair in the reverse direction. Had a resistor been shunted across every diode, their purpose could have been presumed as reverse voltage equalisation resistors.
The analysis is inconclusive.