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"No matter where you go....... there you are"    Buckaroo Bonsai

No matter which switch is thrown first... the filaments will come on FIRST !      Subsequently, when the second switch is thrown...

the B+ will come up.   

When all is powered up.... the next switch (either) to be thrown will turn off the B+ before the filaments are turned off with the last switch operation.

ELEGANT !!!

source: 1941 "The Radio Handbook"  8th edition.