my friend and i have two disc plows.
the later model MF64 (red) with two disc, bent frame plow, only one i have ever seen.
the other older model straight frame with two disc, the tag on it Ferguson (Same model number as yours).
I have an original owners manual with the gray one.
Both are great old plows, will roll right over roots/rocks, etc: either kicks them out or rolls over them, main thing, they just keep on going without tearing anything up,
I am sure they were great tools to have before the automatic kick back plows came along. Otherwise, you would hang up on the roots/rocks and break/bend something on the plow or tractor.
I would say they are rare as you do not see many of them for sale,
The last series of MF disc plow made uses staight pipe, but has a welded angle joint. This type plow frame would have been much easier to manufacture than bending that large piece of main frame pipe.