I tried to find the nuts on the manufacturer’s website but the product code doesn't seem to exist; the nearest I could find were BN 40079 https://eu.shop.bossard.com/ch/index.cfm?app_page=0:31001:30002:11670:1 which should be the correct thread (1.25 mm metric pitch).
Comparing the supplied nuts with some standard M8 nuts which DO fit the M8 threaded rods, you can see the supplied nuts are shallower, so I suspect they are actually lock-nuts (not standard nuts), and they may have a fine pitch metric thread (1.0 mm pitch) or possibly a SAE thread, instead of a standard pitch (1.25 mm) M8 thread. (I don't have any tools to actually measure the threads.)
(Supplied nuts on the left, standard M8 nuts on the right. If you look very closely at the supplied nuts, they are stamped "H" and "04" - but I don't know what these markings mean.)