Actually, I have an open coil Cold Sweat with double screws and it cost me a bit extra. About as much as a covered version.
Strange.
I've had that three times as an extra now and its never cost me more
Edit: And I've never had a 7 varient, but I could swear that last I checked it was the same.
So far as I can tell, tim considers the material costs in such variations neglegable and adds cost for extra work (potting, ageing, heating and cooling, camo-ing, "going at it with a belt sander" etc)
Its a major reason sigle coils are so much cheaper relative to HBs than normal.