My 1976 Jonsered 621 had a chainbrake on it. CSA [Canadian Standard Association] required them at the time , even though they made law not until 1981 that "all" gas powered chainsaws require a chainbrake in Canada. Stihl still managed to sell 090AVs up until the early 1990s in Canada without chainbrakes.
If you look at your 621 you will see a plug on top on the crankcase halve behind the top handle bracket. Here a plunger was mounted activated by a metal tubular loop chainbrake lever, pushing down on a pad which contacted the clutch drum. Also a ground wire was built into the brake pad to cut out the ignition when a kickback happened.
Many loggers including myself disconnected this wire because accidental activation of the chainbrake hitting brush etc would cut out the ignition causing the saw to backfire. Even moisture like snow around the clutch drum would short out the ignition. Not a great idea but the chain brake was better then nothing. Not until the 910 came out in 1979 did Jonsereds get the "brake band " locking chainbrake .
Willard.
