According to vam world there 3 different reverses: C4, C4/C3 and C3 heres the link http://www.vamworld.com/C4-C4C3-C3+Comps
The last coins to use the C3 reverse were in 1903. All 1904-dated coins, with the exception of a couple 1904-S, which are C4/C3, have the C4 reverse.