The Mk1 military units w/ the dovetail cut for rifle sights is just about impossible to find. It was issued in low numbers and I don't think very many if any police picked it up. I have never seen one for sale at any price. I haven't actively been looking for one, but I do keep my eye out.
The version with the bead sight is a little easier to find, but still pretty rare. It was a semi commercial unit that was primarily sold to police. It was also used to some degree or another by the Air Force. I say the Air Force as I have seen pictures of them in use. It may have been a special order that only was of limited issue. I believe Remington went to this format because the rifle sights on the MK1 would fall out. If you check on pictures of the MK1, many have the front sight missing. In fact, there is a fairly common picture of a Marine who has a MK1 in a Sage stock with no sights.
I have one of the bead sighted units. It is not for sale, but I will trade it for a MK1 version.
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It is very simple in construction. Basically it is stamped sheet metal with a bayonet lug w/ sling loop brazed on. It has 3 bolts, one of which ride in the groove on the standard 3 shot Rem factory magazine extension.