William Evans ledger book is very clear. As Wm Evans said In their e-mail. Orders were taken. Once an order was sent to the factory it was "serial numbered." It could then be up to three years before it arrived back at the store for delivery. Here is the ledger book for my Wife's William Evans (subject of a separate conversation). And that would seem to make sense. Most of these firms were selling hand-made high-end guns, not mass market guns. A formal order entered in a order book would seem logical before any work on a gun began.
If I connect an EM Riley SN with a date in the future, I'll emphasize that my estimate is the date it was numbered....not finished.
