The B+ voltage looks to be 185v DC. The voltage divider, 560k and 270k will drop it to 60v DC. Measure the voltage at the junction of R36 and R37, it should be around 60v DC.
You can use any capacitor over 160v dc. I like Polystyrene in this spot. You may try several values from 470pf to 1500pf to see what you like best. Silver Mica is brighter.
The configuration of C16 and C21 could be improved. C21 being an electrolytic will not have the best sound or response. Changing that capacitor to a Polypropylene film capacitor will give the mic a better sound. Try the mic with and without C16 to see what you like better.