Also, the early "cartridges" where quite often extremely simple in construction .i.e. a simple  one piece machined brass "tube" closed at one end, with a machined extractor groove. 

Of course the "cartridges" changed along with the cap technology. 

The early "cartridges" used paper caps. the newer type of  "cartridges" use the" plastic" caps. 

Of course, this type can be extremely complex in construction  being usually 2 or more parts, the case bottom, with the "prima" the middle section and the piston (all usually machined out of brass/steel).  

At last count, there must be at least 20 variants of "cartridges" out there! unfortunately their price reflects their complication level, At least $1.50/$2 dollars each! (depending on the supplier/manufacture) 

The "cartridges" appear to be machined on an cnc driven lathe. 

Swiss watch technology for a toy! so, unless someone out there has access to an cnc lathe and wants to make some..............Japan  might be the only source! fortunately, the "cartridges" are reusable, (if properly cared for) a properly cared for "cartridge" will out last the "modelgun!"

The early caps where just about  alike our roll caps with two differences, they are far more powerful and are sold boxed on flat sheets of paper. 

Typically this type is used just on the older types of "model gun" i.e. the mgc68  all metal "model guns" the mp40, sterling, Uzi and others.

 Possibly Hudson still uses these "paper caps" in their ak 47s etc.  

The other type are the" plastic" caps. The powder, red phosphorus? is placed into a soft flexible plastic carrier ( typically  5mm to7mm in diameter ) 50 to a strip, 100 in a box.  

The color of the plastic, ( gold, silver etc.) denotes the caps power level, loudness and special effects (sparks, infra-red?) .  

As far as I can tell, a fireworks company? Kane? happens to be the only current manufacturer of the caps (the silver color- plastic type only) The "gold" type have the "Kane cap" markings on the strip ( on the "MGC" brand). 

Perhaps, I will find another source for more types.

Although, Marushin has just re-released some older models, I believe that the "modelgun" days are limited due to the air-soft movement. 

After all, why offer limited production/slow selling "modelguns" when you can make more money off of the air-soft market?