Well, I just took the initiative and called the company to talk to them myself. The plugs pictured on their website are all plugs that they can mold directly onto a cable for one of their customers. The plugs are not sold as stand-alone pieces, so there's no hope there for DIY Sennheiser replacement plugs.
The #7 is the one that Cardas used on their first run of cables. After that, Cardas had the company make up a custom mold for the plugs on the Cardas cables, but Cardas "owns" that mold, so no one else can use it w/o their permission.
Having them make up a custom mold costs $7,000-$15,000. The cost of having the plugs molded onto the cables is not insignificant, either. I can definitely see why Cardas charges $150+ for their Sennheiser replacement cables.
Obviously, it can probably be done for cheaper, if Jan Meier is selling an Oehlbach Sennheiser replacement cable for $55. Of course, that cable doesn't feature patented Cardas wire technology, or the very nice Cardas 1/4" plug.
-Jason