I'm messing with the caliper as you suggested Misfit 45 and the fuller seems to be between 25 and 35 thousandths until it begins to taper. The side opposite the markings can still be measured 1.5" from the tip however the end of the blade is definitely thicker than the thickness in the fuller checking both across and along. There doesn't seem to be any more variation around the "weld" then any other part of the blade. I haven't checked the variation on the regular UC M1 though. Not sure it's necessary.
It just seems odd that the mark is on both sides and the spine. I have some other bayonets with such odd marks over an area so perhaps it from corroding a certain way at one point in that area or manufacturing?
I know what you mean about the UFH M1s SKIP. Mine is like that as are all the others I have seen. Seems there quite a few of those around lately. I think it was a source from Mr. Cunningham detailed that as well, but only on the UFH bayonets. Though again, my thinking perhaps it was messed up setting up the machine but good enough to allow through.
I showed it to my Dad who has had a career in maintenance and is a very good welder and he's iffy on it. His thought are that it's odd the marks being all the way around and he pointed out a shade difference around that area. He said if it were welded the person did a very good job with it.
The thing is, unless it was repaired in service, why would someone bother to do such a good job with it or why bother even during the war? As far as it being done after being surplussed the only way it garners more value is if someone knows what this particular variation is. A regular M1 is only worth 75-125.
I don't know if it adds anything but I just noticed that the spotty appearance of the darkness, maybe finish?, still in the fuller only seems to begin after that area. Adds to the simple corrosion theory. However that might have happened with the amount of preservative, position, and moisture. The secluded deeper "pitting" seems similar to marks on other items that have been in cosmoline for a long time.
Another long post but I want to get across as much information as possible to try and reach a conclusion. I also thinks it is making a very interesting case study.