Since Costanzo wrote his excellent volume 40 years ago, additional research has determined that the WaA66 proof was an acceptance used on pistols newly manufactured by BKIW. Others will correct me if I'm mistaken, but I believe this proof is found only on pistols with the s, t, or u suffix. BKIW was a private firm that went into P-08 production in the late 1920's using the tooling that the Erfurt Arsenal used during WW1.
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