I see the letters "SOLI" which could be part of the phrase Soli Invicto Comiti" found on Constantine I coins, and perhaps others.
Don.......
PS: Those same letters might also be read in reverse and appear as ITUS, like in the last four letters of certain kings.
PPS: You may wish to rotate the image (in #5, above) by 180 degrees, to get proper alignment for the coin.
I took the liberty of editing your photos a little. Hope you don't mind. It also appears that there is some lettering on the front of the coin but I can't make it out. Click twice on the picture for a larger version.