I don't have any parts price sheets handy. ....Does that part # appear on any early price sheets, or had it been superceded to a newer part number?
The #1705784 Sheave was used on the Clinton engine on the following Bolens/ME front-tine Mustang tillers:
The M16A Super Mustang(1956-1957), M16B Mustang (1957), and M16D Heavy Duty Mustang (1957) tillers had two speeds for the tines, and they were changed by moving the belt on the two-groove pulleys.
It is possible this pulley # was also used on some of the M16 tillers, but the earlier tillers were single-speed. ....I don't have manuals for all of the early tillers.
For 1958, the engine pulley part number was 1708492.