Good morning HH: well first off there is no book on the market that compares the two.
The Md 21 is simply a better design, structually and mecanically better. It was a "custom order" gun not an off the shelf gun like the Md 23.
The Md 21 was built by master craftsmen in New Haven. Where as the Md 23 was made in the Winchester plant in Japan. the Md 23 is a nice gun, shoots fine, but certainly no comparsion to the Md 21. While some 23s came with choke tubes and are very versatile compared to a Md 21, a Md 21 is superior in just about every other way.
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