hi @TimG,
As I told before this ammunition has not been produced for many, many years and these items are much thought after collectors items in germany. The pen witha similar cartridge system in 16,5mm was only offered for a short time in germany for civilian use. The factory had problems with the cartridges and they were removed from the market. They tried to sell these to the military in other countries. I don't know if there have been any export contracts. They are off the market since about 30 years, but are simetimes offered as collectors items. There is a very nice film on Youtube where a US guy explains and talks about the Heckler & Koch 19 x 36mm launcher-device. It seems that it does not fit under any US regulations. I could not find that film when I searched for it yesterday.
Bellifortis.