Originally posted by Royalan The rings that both these listings refer to as missing are a knurled plastic ring that rests on top of the camera shutter speed dial. The prism then attaches to the camera and indexes with the shutter dial over this ring. The ring is not necessary for the prism to operate, but is there to make it much easier to change the shutter with your left hand thumb while looking through the eyepiece. I think they are hard to come by and can be pricey - I recall seeing one sell on the Bay for as much as the prism itself, but usually sell for around $35. I would bide my time and get a prism with the ring included.
The ring was included as original equipment with the meter prism. Anyone who is selling a meter prism without the ring is not being honest.