I recently got a similarly shaped mark on two negs out of 15 films from my Bronica GS-1. Very similar shape and size - but more diagonal. They appeared in the same place on a consecutive two of four negs taken at the same time with the camera unmoved, on a tripod and with the same lens. (I was using different filters etc.) The scene was in Scotland and very similar to yours Tom - big sky landscape etc. It's definitely higher density on the neg and looks to me like a slight light leak or perhaps sun catching a filter ring or something and flaring back into the lens. Of course, the best of the four negs was affected - well we knew that, of course!
The good news is that his neg was taken early in the trip and was on the first or second film and those two negs were the only one's to exhibit the problem. I'm going to ignore it until or unless the problem arises again.