What year did Neumann U87 switch from 24v to 33v supply (zener) for the FET circuit?

I would like to determine at what time did Neumann switch to the 33v Zener, because of the noteable difference in sound, IMO. So far I have found a schematic dated 1975, however I also found a post on gearspace where someone shows some early u87 with serial numbers, and looking at the PCB, it seems there is an extra component (I assume a current limiter) on mics that pre-date that. here is the pic (hopefully it's not a problem to crosspost) Can anyone help here, by checking their U87, or... Read more