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Joe_G.You will not find a definitive answer here or elsewhere. When it comes to Old Judges, there are many rarities in the set, many without explanation.
In the case of Anson, he was issued two poses in 1888, one in uniform and one in street clothes. His street clothes version was re-issued in 1889 (and 1890) with the 1889 examples being most common.
Do a search and you'll find interesting background information about the Anson "in uniform" portrait. It has been called the Wagner of the Old Judge set as it is rare and has a bit of a story behind it (One of several tricks Goodwin pulled when swapping Anson's head onto another players body).