That serial number is a real head scratcher. May of been a factory "salesman's sample", or something like that. Probably worth a factory letter.
The barrel isn't a full-lug, but it does have the "heavy barrel" (HB) profile. Traditionally, the Model 18 had a standard barrel profile -- a lighter barrel and a slightly different "pinched" frame. (Looking more like the Model 15 of the same era).
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