H and M are often confused on fender tags, especially tags that are rusty and hard to read. The placement of the code on the tag usually clarifies which code it is.
So are you saying your tag has both H25 and M25?
By '71, heater delete cars would be pretty scarce. I don't think many of us were even aware it was possible let alone coded on the tag. Seeing a pic of the tag with the code would be a first for a lot of us. Any chance you can post a pictture of it?