A two diamster bullet is caliber size at the base and bore size towards the ogive, most are divided at a cannelure. How many diameters I don't know, but two if you omit the taper. This was in deference to the short throat on the .264 and later the 6.5 RM. It was an effort to reduce pressure but caused problems for reloaders who were not aware of this and switched components. Bob Hagel was the first person I recall to write about this in Handloader magazine (can't find the article though). It led to the rumor that the 264 produced erratic pressure which didn't help it's popularity.
The Hornaday 160 was a two diameter bullet that may have gotten changed to a conventional design, I'll have to measure them again. I was not sure about the 129 and 154s though, but don't have any of the old 129s around to measure.
