Gunbarrel--
That's a very interesting find. I don't believe it was a unit-directed modification, but at least some individuals in Wilder's Brigade modified their rifles by adding some sort of sling ring opposite the lockplate. On p.32 of "Blue Lightning: Wilder's Mounted Infantry Brigade in the Battle of Chickamauga," by Richard Baumgartner, there is a photo of one such modified rifle, SN 5345, carried by a Private in Co. I, 123rd Illinois (one of Wilder's regiments). I will see if I can add a scan of it. I have also seen other photos of similar Wilder Brigade modifications. Of course without a direct serial number match it doesn't prove anything, but it sure puts a thumb on the scale.
Regarding the local gun shop, if by "obscure side rack" you mean things the proprietor does not particularly value or that perhaps even embarrass him, you might want to mosey on in and save him from it. I can only imagine how ugly it must look next to a pristine row of black semi-autos.
Good luck!
--DJ