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Manufacturer dimensional interchange
Boston Gear 721 appears in the same manufacturer-published cross-reference row as Electra M21 at Match installed ratio (5:1–100:1) · 2.06 in center distance.
Boston Gear P-8565-BG publishes 721 as the 700 Series dimensional-interchange target for solid output shaft — non-flanged reducer (basic unit) at 2.06 in center distance. The table establishes dimensional interchange; gear ratio and application rating must be matched separately.
The manufacturer publishes the two models in a dimensional-interchange table or equivalent dimensional comparison.
Electra M21 and Boston Gear 721 are tied by the published source at Match installed ratio (5:1–100:1) · 2.06 in center distance.
Dimensional alignment does not by itself prove identical torque, thermal rating, service factor, ratio, accessories, or application suitability.