Johannes Adler was a dealer, and the company bought their musical instruments from a variety of suppliers. Obviously they bought at least some of their saxophones from Max Keilwerth.
Alto 5020
Alto 5020
Eb Alto
Manufacturer: Max Keilwerth
Model/Stencil Name: Johannes Adler
Serial #: 5020
Finish: Silver plated
Source: mikkoch01 on eBay.com
Tenor 5631
Tenor 5631
Bb Tenor
Manufacturer: Max Keilwerth
Model/Stencil Name: Johannes Adler
Serial #: 5631
Finish: Nickel plated
Source: kiekertraudchen on eBay.de