You are getting a lot of contradictory advice. If that ring that you show in the second image is threaded and fits the threads of the lens, then that is a retaining ring that holds the lens. The wider bit with the slot cutouts for a wrench mounts at the back of the lens. A spacer is a quite loose fitting ring that is larger in diameter than your original retaining ring you have and loosely fits about the lens in front of the board. It has no threads nor a shoulder. The hole in the lens board should be the outside diameter of the retaining ring so that the depth of the retaining ring goes through the board and inside the spacer. The lens is held by the shoulder of the retaining ring with the slots to the back and on the front of the board by contact with the spacer.
As you have discovered it is the structure of the lens that may allow it to mount without the spacer.
Sadly the required bits only seem to come as a pair on the net but are about 20 dollars.
Perhaps knowing the outside diameter of the longer bit on your retaining ring you can find a bit of tube or pipe to fit and cut to length.
ID 64mm (or 65mm, loose is OK), OD 67mm(or larger), length 6mm.