that is interesting, it does appear to be struck through something, can't really think of anything else that would cause something like that but other will chime in i'm sure...
i agree wiht numismat, but also would add a possible laminet peel.. but 70s where mostly copper.. and that would mean something else was in the die at the time of planchetting...highly unlikely not even a peice of sodder. it ripped the undershell of the coin exposing it.. well id have to examine it myself ...
From the pics, I would say a retained strike through. However, it would be best to see more pics of it (a bit sharper).
