Somehow I knew in my rush to post that these rings came in various outside dimensions, I got it a bit wrong re series and dims, so this is a correction.
A series - 67mm
P series - 84mm
Z-Pro series - 100mm
I have found a 77mm thread P series and the land ( free flat area around the rim ) is 5mm. Probably a bit shy if mounting anything heavy. So for mounting 77mm threaded lenses I would look for Z-Pro adaptor rings. This would allow sufficient land for them to be drilled and screwed or popriveted together for ultimate security.
Don