I am not convinced that the material outside the dotted border is not the result of a garbled double strike rather than an overstrike. I do not believe it is a Christogram and suggest the style of the coin suggests production 50 years before Constantine or at least copying a type of the Gallic Empire period (a barbarous radiate). You will need to find another, more clear, example to establish a new type. Proving beyond doubt a early use of a Chi-Rho would be a wonderful find but I don't believe this is at all certain.