here's my theory on the "3/4 race cam"....

A relatively hot street cam has somewhere in the ballpark of 270 degrees duration....maybe more, maybe less (depends who you ask)...anyways,

...the "3/4 race cam" has 3/4 of 360 degrees duration (adv) = 270 degrees

Just something I thought about...makes sense to me...but then again, I wasn't around when the mysterious grind hit the market.