Thye rear sight is actually part of the rear toggle link on the original 4" guns, and most all the Erfurts, 4" and 8", had the cut-away for the Artillery sight. I don't think that we can tell if the gun was originally an Artillery via the serial number. It may well have been an artillery but was later converted when it was double-dated. Never heard of the Germans dove-tailing a rear sight in, however.
But let's see what the more learned collectors have to say.
dju
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