I am trying to replace an old Hotpoint 2-wire t-stat with a Honeywell digital t-stat. The wires are both red and there no letters anywhere, either on the wires or the unit. One of the wires does have a dark brown or black insulation/casing. How best to do this? Or isd it not meant to be, given the existing wiring system? Thanks!

With only two wires it does not make much difference. Attach to R and W on thermostat and have fresh batterys installed. It may or may not work.