Centennial Serial Numbers
To my knowledge, #105 is the earliest one seen, at least in the last few decades that I have been able to research, in photos or with some proof. I have not heard back for sure if the Springfield museum might have an earlier one or not. If they have one, I would assume it is #100-104, or one of the airweights.
Here is a copy of the #105 S&W letter stating the serials start at #104, just so you can see the reference for my comments. I also have the books and your correct about what it states as well. Like most researchers, I have found discrepancies in the books, as well as the letters, so I dont assume either is gospel until I see the physical proof. Just take it for what it is. I have learned the Centennial information available seems pretty out of wack with what can be proven "in the wild".
Here is more detailed information from Mr. Jinks from back in 2012:
[FONT="]"There was only one .38 Centennial sold in 1952 and it was serial number 104. The production records indicate the following; November 1952 the factory finished 7 units all airweights, the first steel unit was finished in December 1952. In March 1953 the factory finished 8 units and full production began in April 1953 with a total production of 639 units. I hope that this helps. Roy"[/FONT]
So the Airweights were maybe assembled first but serialized farther down the line and if there were only 7 they might have been given as gifts or testers or just sold later. Centennials were very poor sellers in the beginning. The earliest Airweight I have seen is serial 4,047 but I didnt look very hard for those.
The rumor I always heard was the serials started at 100 because it was the 100 year revolver, and that to me makes sense and does track with what has been able to be proven with actual examples. I have no 'positive' proof of that, just an absence of proof to the contrary and I like the 'symmetry' of the idea.
If anyone has any proof of existing examples earlier than #104, please share.