In my opinion you're correct. I haven't seen them like in your example yet, but for me this seems like a repeat sign.
Usually they have a dot on each side and are often found in drum parts for example:
They are called 'Simile marks' and can also have more than one slash. One slash would usually mean to repeat the previous measure, two slashes would usually mean to repeat the previous two measures and so on....
So In this case it seems like to just repeat the last three measures. First because of the three slashes... and also because it's spread over three measures.
This notation isn't in Gardner Read's "Music Notation," 2nd edition. But your guess is reasonable because
each diagonal-line group is three bars long
what precedes the diagonal lines is also three bars long
no other meaning could be given to something that affects four different percussion instruments
it is the multi-staff analogue to simile marks.
I think it's clear from context that it means repeat the previous three bars. It's not quite standard notation. More the sort of thing a composer might use as an indication to his copyist, or a film composer to his orchestrator.