Having a crane at the truck is not that odd in wooden masts. A bit less common in aluminum.
I may prove wrong in the assumptions I make below and I am likely to be answering a question you did not ask, as I can't see anything special in the block brand choice itself.
The counter-sinks on one hole forward and one aft cause me to infer that there's counter-sinks for the other two holes on the other side and space in between what's either two tangs or an upside down square U section. I'd then expect the un-counter-sunk sides to the threaded. This way you can use a flathead ss bolt to hold the blocks (inner holes) and head stay and back stay (outer holes). Add a shackle to the blocks only if you need to create some separation between the blocks and the stays.
G'luck
Ian
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