I have just finished an East German book (source details below) on Hugo Junkers career.
The text has references to Junkers Project numbers which I had never come across before (probably familar to Junkers aficionados !).
For completeness, here are Gunter Schmitt's cross-references of project types:
Junkers Project 1 - Junkers J3 - first light metal aircraft
Junkers Project 2 - Junkers J5 - small light , low-wing cantilever monoplane in three planned versions designed but never built
Junkers Project 3 - Junkers J6 - high-wing cantilever monoplane from August 1917. At the wars end the protoype was half-completed at Dessau
Junkers Project 4 - Junkers R1 - four engine giant monoplane heavy bomber project in two versions, both low-wing.
Junkers Project 5 - Junkers R2 - twin engine giant monoplane heavy bomber project.
Junkers Project 6 - Junkers R3 - four engine giant flying boat project.
Junkers Project 7 - Junkers J12 - four seat passenger aeroplane project.
Junkers Project 8 - Junkers J14 - ten seat passenger commercial project.
Junkers Project 9 - Junkers R4 - three engine giant monoplane 1920 project.
Junkers Project 10 - Junkers JG1 - four engine project for a mid-wing monoplane, a 1920 Schulterdecker.
Junkers Project 11 - Junkers Junkerissime - large land and marine aircraft project with four engines for 60-64 passengers from 1921.
Junkers Project 12 - Junkers J1000 - the design named Riesenente (Giant Duck!), 1924 flying wing project.
Junkers Project 13 - Junkers Ju.X - transport seaplane project derived from the Junkerissime in 1928.
Junkers Project 14 - Junkers Ju.Y - 100 ton transport project.
Sources:
Hugo Junkers and his Aircraft (Gunter Schmitt) Transpress (GDR) 1988 ISBN 3344003038
