There were barely 2000 Philadelphia Arms Co. FOX doubles. Baltimore Arms Co. guns have serial numbers from about 1000 to just over 7000. The serial numbers show just south of 20000 Tobins.
Some others to consider would be Forehand & Wadsworth --
1170 05.jpg
Forehand Arms Co. --
3361 05.jpg
Torkelson --
Torkelson 01.jpg
The little H & R hammer guns --
2384 01 Hammer Double 410-44.jpg
Hopkins & Allen --
3PinSideplate 1914 vintage.jpg
Hopkins & Allen.jpg
These Hopkins & Allen sideplated guns appear to have some of the features of the later Iver Johnson doubles --
Iver Johnson Hercules side.jpg