Starting with the '54 'glide engines all 235's and 261's use a cast iron cam and the distributor gear also needs to be cast iron. Prior to this, the 235's and 216's used steel cams and distributor gears. You can use any distributor as long as you change the gear to match.
A steel gear is hard to scratch using a small file while the cast gear is fairly easy to scratch. Another way to tell is to look at the shank under the gear, if there is a groove there it is cast, no groove and it's steel. By the way, you can use a cast gear on a steel cam, but you can't use a steel gear on a cast cam.
