No, it really was an oddball number. No even multiple of 50 or 66-2/3 I'm betting it was the 216 MHz G3 upgrade. (I know it was a Newer Tech one, so that article's mention of 216 seems right.)

Some quick math makes it appear that since the PB1400 uses a 33-1/3 MHz bus, this upgrade likely used a 6.5x multiplier, making a 216-2/3 MHz chip. I guess we're mostly used to 200+ MHz chips using a 50 or 66-2/3 MHz (or faster) bus, so 216-2/3 isn't a number that "feels" right.

edit: Woo-hoo! Corporal! That's the real rank my grandmother was in the Marine Corps.