I looked at the real old Greenfeild i have here. It got something similar to what that rear engine has but on the same side as the one above.

Pics in this thread.
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=29130&#Post29130

I would say looking at the mower pictured above that they all use the same principal.

Slotted disc on the axle. A spring loaded lever with a tab (vertical on rear engined, horizontal on old tractor) that goes in the slot. Other end of lever hooks up on something to hold it out of the slot when you want the brake off.