I have HAD an old C-More quad rail mounted on my AR;

The last time I went to tighten it up I broke what they call the "Lock Ring"

I called them to see if i can purchase another but they don't make them any more and were ZERO help in pointing me towards a manufacturer/supplier or giving me any details about the part (size etc.)
I want to purchase another one but I am not sure where to start.
I tried ordering one from Midwest Industries:

But when it came it is a different diameter
I have taken the ring to 4 local fastener/machine shops and asked if anyone hand information on what they are called, who supplies them, how I can get one... and have returned empty handed.
Does anyone know what these parts are called if I was to look one up online? Or even better does anyone know where I can get the replacement?
If I cant find another "lock ring" that fits then I was thinking I could get away with a piece that bridged the gap between the rail on my upper receiver and the quad rail, tightening on the weaver mounts of both pieces.

This would hold the quad rail from spinning on the barrel nut and somewhat solve my problem... again though, I would not know what to call that piece so I am having trouble looking it up to order.
Oh great Cal-Gun Wealth of knowledge... Help ME!

The last time I went to tighten it up I broke what they call the "Lock Ring"

I called them to see if i can purchase another but they don't make them any more and were ZERO help in pointing me towards a manufacturer/supplier or giving me any details about the part (size etc.)
I want to purchase another one but I am not sure where to start.I tried ordering one from Midwest Industries:

But when it came it is a different diameter
I have taken the ring to 4 local fastener/machine shops and asked if anyone hand information on what they are called, who supplies them, how I can get one... and have returned empty handed.
Does anyone know what these parts are called if I was to look one up online? Or even better does anyone know where I can get the replacement?
If I cant find another "lock ring" that fits then I was thinking I could get away with a piece that bridged the gap between the rail on my upper receiver and the quad rail, tightening on the weaver mounts of both pieces.

This would hold the quad rail from spinning on the barrel nut and somewhat solve my problem... again though, I would not know what to call that piece so I am having trouble looking it up to order.
Oh great Cal-Gun Wealth of knowledge... Help ME!

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