A letter of authenticity or factory letter will tell you when and where your Model 629 was shipped (usually to a dealer or jobber). It will also confirm the barrel length, type of stocks (usually target stocks made of Goncalo alves), and any special features if they are factory original. It will also provide a history of the model describing when the first revolvers were made, etc.
This information is taken from the factory invoice which is all the information that is available.
Bill Cross
S&WHF