If you wrote that a watch has an not original part, your opinion create a doubt in a potential buyer...
This is not fair in my opinion. Many collectors know the quality of my collection and they know how many important for me the condition of the watches. I saw in my life over 500 Enicar vintage watches and if a watch goes to my collection I think that it is 100% original.
I am only one guy with one guy's tentative opinion.
I remember an argument a couple years ago where some who know something about Gallet chronographs questioned whether the hours counter hand could be correct on a particular watch because the center disk diameter was larger than the seconds counter and the minutes counter. Later, it was discovered that all or nearly all MC-12 H watches have a larger diameter circular disk for the hours counter hand when the movement is an Excelsior park 40 or 40-68, but when the movement is a Valjoux 72, the diameters of the center disk on all three register hands are identical.
We live and learn.
I intend on being humble. My opinion can be changed but it is not changed yet. My opinion is subject to learning from others, especially someone who has a database consisting of 370 Graphs of various type or reference.
It is likely, but not certain, that, if there are other SG Mark III watches extant with seven-digit serial numbers, Nico may have a couple in his database. I hope he does not mind being put "on the spot" like this.
Nico once saved me from keeping a "put together" Mark III with a S/N 1262347. The dealer I bought it from was very nice about it and returned my money, including adjusting in my favor for the change in exchange rate and paying shipping both ways. They later sold it at a discount through an auction at Bonhams.
There is difference here in that a high S/N such as 1262xxx can only be the case back of a Mark IV watch. The S/N of your watch is near the cusp of the changeover from the Mark III to Mark IV. Reference is the same.
Can you put up a photo of the inside case back?
I am going to assume that you are a fair-minded person. I am too.
Assuming the best of intentions is always a good starting point. You exposed the serial number of this watch in good faith and so I believe that you do not doubt what you say . . . you believe the case back to be original.
If someone were to purchase this watch from you, they would face the same question that is being raised here, assuming they exposed the serial number in a public forum.
Perhaps it is better to be discussed here rather than for a buyer to stumble across this thread afterward?
I don't see this as being unfair.