With ANY manufacturing Company, when ever there is a design change there is ALWAYS a "fuzzy" area where they stop one model and begin the next. Sometimes parts , stamping numbers, etc. get mixed up and sometimes a Co. just wants to use up parts they have in stock even though the design has technically changed.
While most of the time value doesn't severely suffer, in some instances it actually makes an item more valuable because there are a very small number of them that exist.
I knew a Dentist that was an avid art collector who focused on one particular artist's work. The artist had started gaining popularity and his paintings had skyrocketed in value. When looking at this painting one day I said to him, "you know the painting is hanging upside down"? He looked at me like I had two heads and then said absolutely not! Since it was an abstract it was very hard to really tell. That said, he showed me a picture of the painting in a catalog and while he was correct in that the painting was hanging correctly, the signature was indeed upside down. He got very perturbed and upset and told me he was going to contact the artist to have him resign the painting in the correct orientation. I told him NO! You probably have a one of a kind and it's probably worth much more than if it were signed correctly. After he calmed down and though about it for a few minutes, he then agreed with my assessment. Throughout history, "mistakes" have mostly been able to command more money due to their scarcity.
So does this apply to your M13? I can't say, but I doubt it would really hurt it.
