WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Pins B12, B11 and B10 state the record size.
If playing a 7" B10 only is high, a 10" and both B11 and B10 are high and for a 12" all three are high.
S306 and S307 detect the position where the stylus is raised or lowered, if that is what you mean, those microswitches are readily available from RS components, Mouser and even fleabay.
PS I olny noticed this question as it is listed under Tubes!