51621 is not in the Serialization Book so only a check of the Parker records by a Research Letter request can tell you more. Your gun was made in 1887 and there are records in the PGCA database on your gun. I suspect the D 4 may indict (D)amascus barrels and 4 (lb) for the barrel weight rather than "D4" in Parker nomenclature which would mean a "C" Quality 4 grade Damascus barreled top lever hammer gun in this case. It is very unlikely that the gun is a 12g is built on the massive #6 frame on which is normally found an 8 gauge gun. 12 gauge guns of this era are normally found on #1 or #2 frames. If it is on a #6 frame your gun would likely weigh 12-15 lbs. Post some photos and more can be discerned.
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