The block to gearbox casing mating has 2 x lengths in 1/4" thread.
Generally in the case of BMC bolts & screws, the letters prefix refer to type, i.e. thread & finish. The (4) suffix numbers refer to diameter & length: The first 2 figures represent diameter in sixteenths of an inch; the second 2 figures refer to the length in eighths of an inch.
In this case: HBZ412 is a UNF thread of zinc finish, 1/4" diameter ([0]4 x 1/16), 1 1/2" long (12 x 1/8). I'll let you work out the length of the second example... HZS (zinc-plated) bolts were threaded full-length.
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