Some German pre-war made exposure meters to consider (I've found these on the excellent homepage by James Ollinger):

Electro Bewi Super
Balda Expophot
Kiesewetter Mini Photoscop
Metrawatt Harvex
Metrawatt Temphophot T-4

or meters like

Super Bewi Precision
Drem Instoscope
Zeiss Diaphot

I pressume the "Sunny Sixteen Rule" was quite a, or even the most, common way to get good exposures by that time.