Baltimore Arms Co. -- January 1900 to October 1904 -- manufactured a gun designed primarily by Frank A. Hollenbeck, covered by his patent #643,601 granted Feb. 13, 1900.
There are three different variations of Baltimore Arms Company doubles, the 1900 Model which was available only in 12-gauge and grades A with twist barrels, B with Damascus barrels, and C with some engraving and a choice of Damascus or steel barrels. Half-pistol grips were standard and the Grade C had the option of straight.
For the 1902 Model they added 16-gauges to the line, repositioned the sear spring and strengthened the protrusions on the toes of the hammers engaged by the cocking slide on the barrel lug. The A-Grade (list price $33) got the option of steel barrels, the B-Grade (list price of $46.50) got some line engraving and a capped full pistol grip, and the C-grade got more game scene and less scroll engraving ("Either half pistol or straight grip.
The half pistol grip has our original finish of ebony inletted in the grip, giving a beautiful finish.") Two higher grades the Trap Gun (list price $125) with straight grip, Fluid Steel Barrels ("These barrels are of extra quality and used only on this grade gun.") and engraving of Pigeons and a live Pigeon shooting scene; --
and the Grade D (list price $175) had finest Damascus barrels or Monumental steel barrels (no doubt named for Monumental shooting park in Baltimore), elaborate checkering and dogs on point in the engraving and a partridge.
The 1904 Model got rebates in the frame on each side of the top and bottom tangs and the head of the stock is inlet into these rebates to prevent splitting.
The grades stayed the same but the Grade B got the options of steel barrels and the choice of a straight or capped pistol grip.
Serial numbers I've observed seem to run from about 1000 to a bit over 7,000. 1904 Models seem to begin in the low 5000 range.
Serial numbers under 1000 were those used by the Ansley H. Fox designed gun manufactured by the Fox Gun Co., Balto., Md., U.S.A. Ansley left in early 1900 to become a professional shooter for Winchester and his partners regrouped, incorporated under the laws of West Virginia, and began production of the Hollenbeck designed gun. They moved into a new factory building in 1901, a block or so away from the old Fox premises.
My two-part article on Baltimore Arms Company was in
The Double Gun Journal, Volume Ten, Issues 1 & 3. I also had an article "The Fox Gun Company of Baltimore City and Baltimore Arms Company" in
The Gun Report, Volume 42, Number 9, February 1997.
No Baltimore Arms Co. catalogue I've seen offered 20-inch barrels. I'd be 99 and 44/100 percent certain they were whacked after market.