[dan]

From an email I got from Brassworks this week when I asked about problems getting original accessories to fit the neck (which I assumed was a problem with neck diameter):

Not the diameter, the shape of the ramp on the lip can be problematic. People have had to file or dremmel the ramp on the inside of the neck to guide the quail on.We have to make our own dies as there is not a manufacturer of these willing to correct the problem for the volume Model A guys buy. It is coming up on our list of dies to make.

So the answer is that they're not going to fix the problem until they get around to making a new die, and their current solution is to have you modify the neck yourself.

I don't believe that this is an issue with "normal" caps, just with motometers and quail. So, if you absolutely must have a dimpled, stock-looking radiator and you're not willing to modify theirs, you could just choose to use a "normal" cap.