1927-1953 Selmer Serial Number List from UArk
Thanks to Jim Gray for this Signet model information!
According to information on Selmer's trademark application, the stand-alone "Signet" name was first used in 1947. Stand-alone meaning just the word "Signet" inside the oval logo. The application specifically says that 1947 was its "first use anywhere", which implies anywhere in the world. In comparison, the earliest Selmer trademark application states that the first use of the Selmer name on musical instruments dates to 1886 even though their first US trademark application wasn't processed until 1931.
According to patent office records, the later Signet trademark, with the scroll coming off the 'S' was first used in 1967 . Therefore, early logo = 1947-1966, late trademark logo = 1967 onward.
The anomaly is the logo that has both Selmer and Signet names inside the oval. These were possibly pre-WWII instruments, but there is no trademark specifically registered for the supposed pre-war "Selmer Signet" logo like the one pictured on Steve Sklar's Clarinet Perfection website. Therefore I think it is a safe assumption that the "Selmer Signet" was pre-1947, but the logo with just "Signet" inside the oval began in 1947, and the late style "Signet" began in 1967.
This ought to help with classifying some of the serial numbers and manufacture dates. I've of course also forwarded this info to Steve Sklar too. In the not too distant future, I'll pass along trademark info about Noblet, Normandy and LeBlanc clarinets that ought to clarify dates of the various names and logos used on those brands.