I've heard of these, but never seen them.  I think there's some mention
of it in Bowers' "Encyclopedia of Automatic Musical Instruments", though
I'm not sure.  I've read about it somewhere, though.

If memory serves, the narrow slots above the roll would on the
appropriate rolls increase the dynamic level of that particular note
one level.  If that's not exactly it, it's something along those lines.
I seem to recall reading that these systems were exceptionally rare,
and the rolls for them were equally hard to find.  I'd suspect that
finding a replacement tracker bar would be a quest not unlike the grail
quest of Arthurian legend.

Bryan Cather