Siemens UL 1066 Operating Instructions Manual page 206

The coupler snaps onto the end of the steel mainshaft. The steel band should not be removed when installing the coupler. Also ensure 
that the coupler is oriented properly when installed. Figure 3 illustrates the proper installed orientation of a shaft extension (circuit breaker
shown in the OPEN position), with the tampered flange facing the rear of the circuit breaker..
NOTE
The tapered flange of the coupler must point towards the rear side of the circuit breaker.
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Figure 3