I think we are all partially wrong, even Pertronix.
The 1168 fits the 1954-1962 distributor cam but is for use on a 12v system. There is a special part number for use of this cam-size with the standard 1954/55 6v electrical system: 1168N6 (for negative ground, 6v system).
The 1167 is for the earlier 1941-1953 216/235 distributor cam and the original 6v system (the smaller cam was never used on a 261 and there was no 261 in 1953). The 1167P12 is for an earlier distributor/cam on a vehicle/system that has been converted to 12v.
Check out
Jim Carter\'s years for distributors. Take a look at the years/engines/voltage in the
Pertronix Catalog. The notes for the 1167 & 1168 units are confusing and partially incorrect. GMC units are even more confusing and incorrectly described, I think.
I have an 1168 in my 1960 261 (12v) engine and an 1168N6 in my 1954 235 (6v) engine.