The Bantam was actually the DKW RT 125 that the Allies recieved the rights, tools, and mfg capabilites to as war reperations for WWII. Harley Davidson (hummer 125), MZ (IFA), BSA, Moskva, and yamaha all made the bike post 1948. The BSA bantam is unique as the the BSA designers mirror imaged the original DKW blueprints so they could make the bike right had shift. I think the legend is they literally used a mirror to copy and invert the plans as EVERYTHING is a mirror image of the original DKW. Interestingly enough the bike was already being copied by Royal Enfield in 1938 after the Nazi's pulled the dutch franchise for DKW. The bike was such a big seller in the netherlands that the importers comissioned RE to copy it and sell it. The RE engines were different (lacked transfer ports), but the basic frame design was pressed into war service as the RE/James Flying Flea - a motorcycle that could be air dropped with paratroopers (yes I notice the irony that the german technology was used to directly fight them).
The original design dates back to the 30's and was in production until 1971 in one form or another. Estimates for how many produced are between 250,000 and 500,000 across all manufacturers (BSA was the largest volume producer). Yamaha did not recieve the war reperations plans and such since they were on the wrong side. They actually reverse engineered a DKW RT 125 and produced the YA1 from 55-58.
Hermann Weber was the designer for the original DKW in the 30's.
Bruno Cavali was the importer for DKW in the 20's thru post WWII. After the war he couldn't import them so many italian motorcycle mfgs got their start making retyled RT 125 copies (Mv Agusta, Maserati, etc).
Hermann Weber was the Cheif Designer for DKW and all the designs from the 20s to the 40s bear his mark. This includes some of the cars and some Auto Union and Audi designs as well. Weber was also the head of DKW racing department and laid the ground work for Ernst Deigner and Walter Kaaden with MZ in the 50's and 60's.
I have read that the goverment of japan actually prohibited the import of the western world's bikes...That the gov. turned a blind eye to the newly formed motorbike manufactures importing those 'test' bikes.. As Geeto stated these were then reverse engineered (again blind eye) to help build an expanding post war economy with proven designs that were soon to be sold world wide...