I can tell you that it is in the *range* of serial numbers issued to the 6th Indiana Cavalry and the 6th Illinois Cavalry.

The two closest numbers are:

30563 - in possession of CO. D, 6TH ILL. CAV. in 1864

30571 - in possession of CO. G, 6TH IND. CAV on 07/13/1864.

[Source: Springfield Research Services]