Open end headset wrenches are 1/8" thick, and can be purchased thru many sources.
I have a pair of 40/36mm wrenches for a 9/8" headset.
from Park, a bike tool company.
(and some 32mm ones for 1" headsets.)

the half MM is odd, .. Maybe a pair of 36 will do,
36 is also used on the threaded headsets on my Brompton,
another 1.125" threaded type.