To install the M1917 O-ring and spacer you need:
Step 1. Grease the barrel where the o-ring makes contact and install the o-ring.
Step 2. Install the brass spacer with the flat end facing the o-ring.
Step 4. Screw on the muzzle nut/booster until it’s snug. Push on the barrel and make sure it moves freely in the gun. If they barrel does not move, loosen the booster nut 1/4 turn. The booster nut not being tight against the water jacket wont hurt anything. If you are afraid of it coming loose, remove the booster and wrap the threads in teflon tape for added friction.
If you want to make the spacer a prefect fit (tight on water jacket) with your M1917, grab a file, some emery paper. You may only need to remove .015.