I agree that has always been surprising that no-one has come forward about the bat but it may make a little sense when we think about it.I will never believe no one recognized the bat. I certainly believe the parent(s) of the perp recognized the bat....
One thing that I think of is that back in the 70s I believe wooden baseball bats were fairly inexpensive so bats were not as coveted as they are today, and there was not any real marketing about the different or latest models, like today. Today's baseball bats, including youth bats, are very expensive.
This bat may have been just lying around somewhere and picked up at a park, garage sale, etc., though a recent poster said that a poi was known to carry a bat around with them.
Also, although it seems that someone should certainly recognize the bat, but then again, unless you were on a baseball team, which would be a very small group especially at HS age, and saw this particular bat up close on a regular basis, we probably would not pay that close of attention to the details to what a wooden bat looks like. At glance they all look kind of alike.
The perpetrator probably did not play organized baseball so who would really know what an old wooden bat that they had lying around looked like, except for perhaps the people that were there that day, and maybe a parent but even a parent may not know either.