Manufactured by DIAG GmbH. Landsberger Str. 32, Leipzig, 1921-1928
Initially the firm built bicycle attachment engines of 83 and 101cc, and later built DIAG Type II light motorcycles using similar engines with belt drive and chain drive as an option, along with conventional motorcycles of 175cc to 350cc using engines from JAP and Kühne.
More expensive models had the exhaust gas routed through the footboards, and electric lighting. Many were available with the option of chain or belt final drive.
Type I: DIAG bicycle engine 138cc 4-stroke engine, available with bicycle pedals or footboards. 
Type II: DIAG bicycle engine 83cc
Type V: DIAG bicycle engine 101cc
Type VII: DIAG OHV engine 185cc
Type VIII 175cc sidevalve JAP with Sturmey Archer 3-speed gearbox.
Type IX: DIAG 350cc sidevalve 8hp engine
Type X: Kühne 350cc OHV 16 h.p. engine
Notes. 1. Most sources give bicycle engine capacities of 83 and 101cc, so 138cc may be an error.
Sources: motorräder-aus-leipzig.de, motor-hist-foto.de, wikipedia.nl.