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3657 starter solenoid
3-terminal with insulated base. Notice the battery cable stud is identified. That is because the magnetic coil is powered by the battery stud internally. The 3656 3-terminal ignition switch for magneto ignition and electric start grounds the small terminal to activate the solenoid on the 551.
A suitable replacement would be a 4-terminal solenoid with an insulated base with a jumper from the battery cable stud to one of the small coil terminals. The other small terminal is grounded be the ignition switch to activate the solenoid on the 551 and a push-button switch on the 701.
1961 model 551
1961 model 701