I had thought that it could be a manufacturing defect due to the emulsion application at the factory, but having it in both directions (horizontal on one shot and vertical on the other) seems to rule this one out. It also rules out an x-ray fogging problem for the same reason. The suggestion to process a blank sheet seems to be a good one, but what about taking a shot of that flat type of sky or a plain neutral wall to go along with the blank sheet as a control?

It sounds like an exposure issue if the film orientation was different in the camera for the two shots. Having distinct banding which is the same on two prints from the same film indicates the film itself, not chemistry. What happens if you print a sheet of blank, or plain even tone film, at grade five in your enlarger? Please keep us posted, this is truly strange. tim