1972 Ski-Doo Shop Manual, Page 79Get this manual
Transmission GEAR BOX (Alpine and Valmont)
GENERAL
All Alpine, Valmont and Nordic 640 ER models are equipped with gear boxPower transmitted to the driven pulley is relayed to the gear box because of the common shaft passing through
both assembliesThe method of interconnection and operation of the components of the gear box enables forward and reverse movement of the vehicle