I've been trying for several years to make this information clear on the internet, yet people still continue to use the trumpet serial numbers lists to date Selmer Paris cornets.
There is a separate serial number list for cornets. If you want to date a Selmer Paris cornet, all it takes is an email to [email protected] and they will provide you with the date.
An easy rule of thumb is any cornet with a three digit serial number lower than 600 is pre-WWII. If the bell is stamped with award info from Geneva, Barcelona and Liege, it is pre-WWII. If Barcelona is omitted, it's likely from WWII up until early 1950s, when they discontinued stamping the bell with that information.
The earliest K-modified cornet I have knowledge of is from 1957 - 16xx. The latest four digit serial number is 45xx from October 1982.
Although I'm not completely sure, I believe there was a period of time between 1982 and the late 1990s when they weren't making any cornets.
When Selmer began manufacturing the Concept model cornet, in late 1990s I believe, they used a different serial number system with five digits.