1950's HH Scott Sound Level Meter

[iangomes]

Found this sweet little (not actually so little, it's almost a foot long!) sound level meter for a few bucks at a pawn shop near me. Appears to have miniature vacuum tubes in it and apparently runs off of a 413A 30V battery. I was hoping to modify it to be used as a microphone with an output. Anybody got any information on these? Can't find a schematic anywhere.

Is it a condensor?
Looks omni to me
It appears to have space for two small batteries as well as the 413A... any guesses?

 

[iangomes]

Founds some more pictures, including internals.
Also looks like there are some technical powersupply/battery details but I can't read German.

 

[Joe Bidwell]

Do you know the number of the 1.5V batteries? Thanks!

 

[Bill Briere]

New guy here.

Has anyone determined what the 2 smaller 1.35-volt batteries were and what might work as a modern substitute (with or without an insertable adapter)? I'm selling a Scott 410-A SLM like this (elsewhere), but was unable to test it and am getting questions about these unidentified batteries. The also-required 30-volt battery is easily available, so no problem on that one.

If it's any help, a previous owner of my device had jotted down a note describing the smaller batteries as "RM3 or RM3R Cells 1.35v."