Man, I can't tell you how awesome it is to see builders really studying the manual. I bet your build will turn out really well!
The plate choke in the SEX amp also does duty in the Stereomour. For the 2A3 configuration of the Stereomour, too much current is drawn to use the whole winding (red to blue), so the tap is used. This decreases the inductance a bit, but provides sufficient current headroom. In the SEX amp, you will never need that current headroom, so the black lead can be snipped off. It will have nearly the full high voltage B+ present on it, so some heatshrink is prudent here.
On the schematic, one channel of the actual amplifier is shown, and the B+ choke label here indicates a feed form the power supply.
Down at the bottom of the schematic, the power supply is shown for both channels, and there is a B+ choke label for each channel.
Each B+ choke feed does have its own resistor/capacitor filter, so the 100uF capacitor there is correct.
-PB