These are a customized aftermarket cap. I am unsure what plastic the aftermarket company used originally. I considered at 3 options:
1. Remolding a cap with the "K", this would be best for high volume (tens of thousands) but I suspect only a few hundred at the most would sell even if priced low. The initial cost of creating a mold cost in the neighborhood of $10,000.
2. 3D printing new caps. This was a little more feasible than option #1, but the cost of having a 3D image of the cap created would have been quite a bit. The added cost of having them printed with a high enough quality printer would have made them extremely expensive.
3. Customizing an existing aftermarket cap. This was the only feasible option, and being they are such low quantities the price still ends up being higher than most would like.
IF I had enough interest in these to make larger quantities the price would come down some. But they will never be as inexpensive as the $15-$20 kimpex caps.
I always thought that the original "K" caps cracked because guys would crank the cap too tight in an attempt to align the "K". I suppose they could be made of a slightly different material, but I doubt it. I don't think there were many options in plastic material types (at least ones that will withstand gas) back then.