curious cam sprocket wear- 340

while inside my timing case today I noticed something i had never seen before. Basically the front teeth (double chain roller) of the cam sprocket were being eaten away from the front towards the back. It was the chain doing it. the front teeth were noticeably thinner in a front to back direction, although chain tension was excellent. It appeared to me that the cam was too far forward.
I rushed over to my machinist and he agreed.On returning to my shop i decided to yank the timing case on a 1972 steel crank 318 i had laying around, granted it was a silent link type but when i compared the two cam sprockets, the 318 one was perfectly flat on the back side where it kisses the cam, but the one that is wearing (340) has about a 1/16 inch raised shoulder, i assume that shoulder was holding the sprocket forward and causing wear. what gives?? or am i crazy?and yes i already thought about where the worn away sprocket material might be.
neal