Curt,

Ignore the hot-rod guy unless he really knows 216/235/261 engines.

At some points in time the distributors changed. Some of the changes had to do with how the points were attached. the height of the rotor and cap, and the cam drive gear (also the angle of the vacuum unit but that is not really a change in the distributor).

Unless someone can post here what changes might be important to you, you might want to wait on buying/installing the Pertronix unit until you install the 261 - get a 12v 261 unit for a 12v 261 distributor of a known year.

Good luck,
Tim