Ike,
There seems to be a lot of information as well as misinformation regarding your Guild. I purchased mine brand new in 1977 from a large music store in S. California and have enjoyed it ever since and I think that is the key, you have a fine guitar with a great sound. I have seen many with the dark stain and no "M" suffix after the model number. Mine is a D25M and I was told when I bought it that this is because of the mahogany top, those without it have a spruce top, that is what I was told in 1977. I do know this, I took mine back for some rework to the finish about a week after I bought it and they errantly gave me the wrong guitar back without the “M” suffix and the sound was not nearly as rich. I was grateful to get mine back. With that said, both sounded great and both were very fine guitars. As a side note, I have played mine with many other players over the years that were playing a variety of fine guitars such as Takamine, Martin and Taylor and although I would be proud to own any of them, my Guild over powered most of them and the tone is amazing. At the end of the day, just enjoy what you have, it will serve you well for many years to come.