If you mean the series number is K.... , then the Clarinet is not from 1948 but from the early 1930's (a 1948 Selmer would be N....). As such it would be of interest to people who prefer early Selmers, and if in really good condition it might be worth around £500 (sterling). Personally I think the K's and L's are the best they ever made, but not everybody would agree!
Selmer 100
Selmer 1430P Bass Clarinet.
Selmer 10G Selmer 10G
375 dollars
Any good music store will have French made clarinets and the better models will be made of wood. The most common brands are Buffet Crampon a Paris and Selmer Paris (not to be confused with Selmer USA).
The Selmer Signet clarinet with serial number 11998 was produced in the early 1960s. Selmer clarinets typically date their production based on serial numbers, and this particular model falls within that timeframe. Therefore, as of 2023, the clarinet would be approximately 60 to 65 years old.
950
Selmer sells products designed to help you properly take care of your Selmer Clarinet. One item they sell is a pull-through Dri-Bore clarinet swab. This is used to clean the inside of the instrument. It is also important to have a well-made carrying case with strong latches to keep the clarinet from damage due to accidental dropping or bumping.
2 years
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1960
Can't help you on the value--I would look for a clarinet blog or web site and ask there. The Selmer serial number chart indicates that would have been made in 1942.