This の is an "empty" noun - a noun in form and function but with no content/meaning. It is always modified by a sentence (of theoretically any length, from a single word up) and takes on the meaning of that modifying word or sentence. To put it another way it turns what modifies it into a noun, which can then serve as a subject, an object etc. Here, the sentence マッカーサーが自分で作って見た, "MacArthur tried making one himself" or "MacArthur made it himself as an experiment", becomes a noun, which can be rendered into English as "The thing MacArthur tried making himself", or "What MacArthur tried making himself". This noun phrase is marked with が as the subject of the verb である. The whole therefore means, in an over-literal translation, "What MacArthur tried making himself is Map A". In this sentence it is the first part that the writer regards as new information for the reader. The reader clearly knows there is a Map A because it is (presumably) there on the page, and the writer is supplying the new information that what this map represents is MacArthur's own effort at making a map of whatever-it-is. I would render this straightforwardly as "Map A represents MacArthur's own draft" or "MacArthur's own draft is shown as Map A".
Consider:
バスに乗る [Somebody] gets on a bus
バスに乗るのを見た I saw [somebody] get on a bus
きのう、駅の前で田中さんがバスに乗るのを見た I saw Tanaka-San get on a bus in front of the station yesterday
きのう、駅の前でむかし英語を習った田中先生がバスに乗るのを見た I saw my old English teacher Tanaka-San get on a bus in front of the station yesterday
And so on.
