Colt was actually gone by the 20th Century --
6430 02.JPG
My 1900 J.H. Johnston Great Western Gun Works catalog was still offering the Colt Hammerless Doubles. I says only a few left.
American Arms Co.'s Whitmore Hammerless just made it to the 20th Century --
63 05 12-ga American Arms Co. Whitmore left.jpg
Syracuse Arms Co. started about 1893, but made it to maybe 1908 --
Prototype 01.jpg
Baltimore Arms was 1900 to 1904 --
2243.C C-Grade 08.jpg
Hollenbeck/Three-Barrel/Royal Gun Co. was 1901 to 1910 --
126 Hollenbeck left side.jpg
Tobin Arms Manufacturing Co. was founded in 1904 --
7574 10.jpg
operated in the U.S. through 1909 then moved to Woodstock, Ontario, Canada and operated until WW-I.
Ansley Fox's Philadelphia Arms Co. was incorporated in 1902, built a factory and was finally producing a gun in latter 1904 --
454 08 Right Close.jpg
Then in late 1904 Ansley left and by the spring of 1905 founded the A.H. Fox Gun Co.
Those are a few I can think of quickly. There were some nice J. Stevens Arms & Tool Co. doubles in the years just before WW-I.