Those brake mounts were proprietary mounts as mentioned that only came in IS format, I had to get one for my Z1 QR20s (which had a Z4 serial number) and they were thin on the ground in 2004. I’d say they’re damn near impossible to find now but that Dr Marzocchi guy may turn something up and there’s another goldmine in Germany with an internet site.
I definitely don’t remember Z5 20mm forks, plenty of Z3 and the occasional Z1 but Z5 must have been OEM.
Ah yes QR20. Anything but “quick” and the dangly bits sometimes wound off in transit.
That was the 2003 successor, QR20+. QR20 was literally a 20mm QR mount with a QR 20mm axle, basically what you were left with when the stupid cams or barrel screws disappeared.