Thankfully this fell off at the bottom of the hill.
The bike, a Trek Procaliber 9.5 , the crank, Shimano MT511, 30T steel ring, 52mm chainline, 175mm length.
The MT511 data sheet show the parts, funnily enough.
The question is why did this happen? Did the bolts simply come loose? Do I trust it to use it again? Why will the Crank Arm Fixing Screw not thread on fully?
Replacement on the way, why is it so hard to get details on whether a crank arm has a spider or not?
https://www.rei.com/product/187784/shimano-deore-xt-m8100-12-speed-crankset
https://www.rei.com/product/161405/shimano-deore-xt-1x-12-speed-chainring