Is the example above the type of thing being discussed in this thread? This is a section from the top left of the sheet seen in full below - I upped the contrast to make the marks more obvious. This is a sheet of 8x10 HP5 that I exposed on Sunday and processed yesterday. It was processed in a Jobo 2830 print drum (with internal ribs), rolling by hand in a sink. There was no pre-soak; I used Rodinal 1:25 (16ml to 400ml) for eight minutes; water stop; fix for about 10 minutes; wash; hypo clear; then final wash for 15 minutes. When I saw the marks after the film was dry I washed it again and soaked in photoflo again but the marks were still there.

Very interesting to read Photo Engineer's remarks.
I've done a lot of HP5 8x10 in this tank and never had these marks before. I have always done a pre-soak, but this time I didn't bother, so that is the most likely cause in my mind.
I always use a water stop, but again it is interesting to read that Ron thinks stop becomes more critical as you go up in format. So perhaps the strange marks are from extra development happening due to uneven stopping of the dev. The marks are almost certainly occurring where the film base is in contact with the ribs.
Either way, both of these things are easy to do, so from now on I will be doing the pre-soak and using proper stop bath, at least for large format. Fortunately this was just a demo shot.