That had to be a hoop of crankcase pressure
I've not seen that as far as I can remember
My old 415 HP 1963 M.W. Stocker would blow one or both breather caps off in the burn out with 42,000 miles on the original pistons, the top ring groove had over .011 thousands side clearances That was back when NHRA only allowed OEM stock pistons
I should have had those pistons cut wider on the top ring and used a spacer to tighten up those clearances, hind sight is wonderful, isn't it