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[tvanderbrook]

I don't think it's likely that the two 22.5V batteries produce more than 45 V, but it's a possibility (although unlikely), right? There are battery supplies that power 48 volts from a 9v battery…

I checked the actual voltage from the Millennia once, but can't remember where it sat. I think it was somewhere between 45v and 52v; somehow my recollection is that it was lower than 48.

In any case, slightly sad to find out this is not actually true, however these two mics sound so great I'm not that sad.

Is there a spot I could put my meter leads to see the voltage from the batteries?

[klaus]

The batteries are connected in series:
Insert one end of your probe at the tip of one battery, and the other end at the bottom of the other. Alternatively, take both batteries out of the mic, and connect (+) of one battery to (-) of the other, and then measure at the two free (unconnected) plus and minus terminals of the two batteries.

[Kai]

...Alternatively, take both batteries out..

I'd suggest to measure the batteries in place, with the mic turned on and plugged to a cable. The little instrument in the mic shows the battery condition then.
For exact numbers use your probes under the same condition with the batteries in place.
Only if the batteries are powering the mic for some minutes before measurement you get correct values.

BTW:
The supply voltage to the mic's amp is stabilized internally to 24V by a zener diode (GR1).
So increasing the supply voltage does only change the capsule polarisation, giving some tenth of a dB more sensitivity.
For the amp nothing changes.

The U87 isn't very current-hungry, in fact it's 0.4 mA is far on the low side.
It's doesn't stress a P48 supply.

Regards
Kai