The 5703WB is a super subminiature, medium-mu triode that provides excellent audio performance and has been used by many audio manufactures such as Musical Fidelity in a range of is audio products.
I have an interesting note on these from one of our customers who has stated that he has used these to replace the Telefunken AC 701 and AC 701K in tube microphones, such as Neumann the original M and KM series.
He stated that pin out would need to be changed so you will need to get the relevant data sheets. The only modification that he stated was that he made was in the power supply as the filament voltage needed to be increased from 4 volts to 6.3 volts. The filament of the 5703WB does draw more current, 5703WB filament draws 200mA against 100mA for the Telefunken 701. The result is that it does run hotter so it is worth considering this before you modify your microphone and you will need somebody that is competent to do this.
This is posted on the understanding that it is for information only so you will need to gather the relevant information.
This listing includes many hard to find tubes from manufacturers like Philips, GE, Sylanvia, Raytheon, and more.
