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Manufacturer functional interchange
Boston Gear 842B/843B appears in the same manufacturer-published cross-reference row as David Brown M042(3)BRN at Match installed ratio/application · Boston 800 size 840 · Foot mounted.
Boston Gear publishes 842B/843B for 800 Series size 840 in the foot mounted column of its in-line helical interchange guide. Boston states the 800 Series is designed to be functionally interchangeable with listed drives while requiring verification of ratings and dimensions.
The manufacturer describes the target family as functionally interchangeable with the listed competitor model or configuration.
David Brown M042(3)BRN and Boston Gear 842B/843B are tied by the published source at Match installed ratio/application · Boston 800 size 840 · Foot mounted.
The source still requires ratings, dimensions, ratio, fit, and application conditions to be checked before ordering.