The wires going into my old general controls thermostat are labeled M, T, V, and H. how does this conform to Honeywell's B, O, G, Y, W, R, and Rc on theRTH2300/RTH 221 series thermostat?

I have no idea other than to suggest you jot down which color wire is attached to which terminal of the thermostat & then do the same at the furnace. With that information we can probably help you with the wiring.
Right from the start, we can rule out any connection to B or O.
Do you have A/C? If not, we can rule out Y.

They are all black. No A/C... I think V is black and H is red from previous research.

They are all black. No A/C... I think V is black and H is red from previous research. The M T V and H are factory labels on the old thermostat... general controls catalog #Y270A100, with serial # B8A51

The M T V and H are factory labels on the old thermostat... general controls catalog #Y270A100, with serial # B8A51..

All black, wonderful. Without being there, I'm not going to be able to help, sorry.