CNC arched Z1's came in either 4 or 5 inch travel.
It sounds as if you've got 4 inch ones and the extra 20mm is the bump stop, although I think the stanchion length on my 4 inch Z1's is 110mm I seem to remember.
There's an easyway to tell for sure. Undo the crown bolts and slide the stanchions out of the crown.
Unscrew one of the topcaps all the way out and empty the oil into a container, then lower the stantion as far as it will go into the lower leg and use a rubber band or ziptie to mark this level on the statchion.
Fully pull the stanchion out of the lower leg and measure the distance between the band/zip tie and the fork seal.
This is your max travel.
(btw the difference between a 120mm stanchion and 110mm stanchion will have an almost un-noticable difference on the feel of your bike, especially when you dial in 10-15mm of sag)
Hope this helps, but if I were you I'd just leave it!