Right off, I do not see a Gustav Diem & Compagnie of Frankfurt on the Main(River) but there are a whole host of Diems in Suhl; more than you can shake a stick. But I do find a a Gustav Diem of Suhl-Benshausen, hanging out a shingle @ Mühlstraße 8. Frankfurt on the Main must have been a satellite office?

Serbus,

Raimey
rse