I've been amused by all the bikes that were derivatives of the DKW RT-125. Most recently Yamaha produced a concept bike, the Y125. This is an imitation of their first motorcycle, the YA-1. And that was a copy of the legendary DKW RT-125.
Just compare these pics:
The 2011 Yamaha Y125 Concept bike
The original 1955 Yamaha YA-1
And the ORIGINAL 1939 DKW RT 125
For the record, the 1939 DKW RT 125 was a great design by Hermann Weber.
It appears that after WW2, the design of the DKW 125RT was open to anyone who wanted it. Some manufacturers aded it to their product portfolio, but other companies were started with that very bike as the first product.
The RT 125' is a German two-stroke motorcycle made by DKW in Zschopau in the 1930s, IFA and MZ in the 1950s and early 1960s, and DKW in Ingolstadt in the 1950s and 1960s. "RT" stands for "Reichstyp" or "National Model".
In the 1930s DKW pioneered the Schnürle two-stroke loop scavenging process to dispense with the use of a deflector piston and improve efficiency of the combustion chamber. DKW also developed a highly efficient arrangement of transfer ports. These two features were included in the RT 125 to great commercial advantage. Competitor companies such as Adler and TWN copied the adoption of flat-topped pistons and strove to develop equally transfer port arrangements without infringing DKW's patent.
The RT 125 is probably the most copied motorcycle of all time. After World War II the Soviet Union took plans, tooling and even several dozen personnel as war reparations to MMZin Moscow (later transferred to MMVZ and SMZ) and to a factory in Kovrov, and produced copies of the RT125 as the M1A Moskva and K-125 respectively. WFM of Poland made a modified version of the RT125 (under SHL 125 and Sokół 125 brands), developed into 125/175 cc family motorcycles, produced until 1985. RT 125 plans were also taken to the United Kingdom where they became the basis of the BSA Bantam, and to the USA where they formed the basis of the Harley-Davidson "Hummer". Later Yamaha in Japan copied the RT 125 as the basis of the Yamaha YA-1.
Here is an array of shots some of these copies.
An American 1948 Harley Hummer
The English 1948 BSA Bantam
The Soviet 1946 Moskva M1A
An Italian 1950 MiVal 125T
A German 1941 NSU 125 ZDB
An Italian 1954 Maserati L/125/T2 (Italmoto). Its hard to see in this shot, but the Maserati engine is just like all the others. The frame is a bit more modern.
The first Italian Moto Morini, the 125 Turismo. Thile the top end is the same, the gearbox appears to have a different from the DKW.
The first Italian MV Agusta, the 98 Turismo. Looks like a DKW top end but again with a different gearbox.
An Italian 1948 Sterzi 125 Turismo
The first Japanese Kawasaki Motorcycle, 1955 Kawasaki (Meihatsu) 125
A Norwegian 1954 Tempo
The Czech 1949 Jawa 125
The Hungarian Csepel 125
There are lots more, google these up if you want to see more bikes closely related to the DKW RT 125.
James Comet 125
Sokół WSK M01, M06
SHL M02, M03, M04
Royal Enfield WD/RE Flying Flea