As @ad180 pointed out, the boxes we see are power supplies for tube mics. Each type of mic had its own power supply, as power needs differed from mic to mic. And with some mics (notably the M49) the power supply is where you changed the pattern. P48 was introduced for solid state mics in 1966, but it wasn’t initially ubiquitous like it is today. The U87 could run either on P48 or via 2 22.5V batteries. My guess is in 1970 CBS was running them with batteries. Neumann Models U 87 and U 87 Ai Now…maybe that’s not the problem we see/hear at all. Not sure.