To install the Vickers O-ring and spacer you need:
Step 1. With the barrel removed install the o-ring into its seat inside the water jacket. It looks like it wont fit, but it will.
Step 2. Grease the barrel where the o-ring makes contact and reinstall through the o-ring.
Step 3. 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 Vickers, grab a file, some emery paper. You may only need to remove .015.