The Aluminium boom and mast was an option on the early 38's and 42's. No idea who manufactured them for Marshall, but I never received any literature for the boom when the boat was delivered.
I used it to lift a small motorcycle aboard. Probably weighed 150lbs or so and wasn't much of a strain on the gear. I was more concerned about the capacity of the plastic components on the block and tackle setup. Although in retrospect, it looks like typical rigging components found on a 40' sailboat. So probably capable of handling greater loads than I was lifting.
Careful . . .I Have a Generator and I'm not afraid to use it !