Co's A and B of the 42nd Ill. Inf. were at first armed with the Colt RR.

There were a dozen or so in the 73rd. Ill. in the 2nd Qtr of 1864.

And the 9th Ill CAV. had over 350 of them in the 2nd quarter of 1864

Source is "Arming of the suckers" by Ken Baumann

There are other Ill units that had a smattering of the Colt RR at times - one here - 5 there - etc.

Hope this helps - There is a great print - "The bayonet or retreat" by Andy Thomas showing the 37th Ill. Inf. at Prairie Grove Arkansas - and he correctly shows the flank company with their Colts.