on a short nose cam it will have the t-gear and a different style washer/bolt.

if it has the ecentric,you should see it in front of the t-gear looking into the fp hole.

IIRC a magnum has a thinner t-cover and this is why a fp ecentric will not fit behind the mag cover.

but by using a pre-magnum t-cover and the hughes fp adaptor you can run an ecentric on the short nose cam.

if the roller cam pre-mag engine had a carb it had a long nose roller cam for fp ecentric. if it was TBI it had the short nose roller cam with no ecentric but still had room in the t-cover for it. pre-mag TBI t-cover got a fp block off plate.

the pre-mag roller cams,short or long, will have the bearing journal drill for oil the rockers thru the heads.

a magnum roller cam will not. and it can not be used in a pre-mag LA engine block.

here is a pic of the short roller cam nose with a tape mearsure for referance.

make note of the flat t-gear washer and bolt. this is different from the ecentric and its own washer/bolt.