Well, after ordering my Mugen high-pressure radiator cap and finally receiving it this week, I went to put it on and the darn thing just wouldn't fit cuz it was too small. So I checked the King Motorsports Mugen site and the serial number for the cap listed there was the same as the number on my cap's box. So I tried to get it to fit one more time and no dice.
I have a Toyo Radiator and so I figure that if you've got a Toyo radiator you're screwed and the Mugen cap won't fit. Perhaps the cap only works for folks with the Denso radiator. Lucky buggers.
Oh well.
Just thought I'd post this here to save anyone else the time and trouble.
BTW - that cap is the exact same cap used for the Civic and Integra as well.