Protocol upgrade mechanism - HTTP
sec-websocket-version request header specifies the websocket protocol version the client wishes to use, so the server can confirm whether or not that version is supported on its end.
... sec-websocket-version: version version the websocket protocol version the client wishes to use when communicating with the server.
... response header if the server can't communicate using the specified version of the websocket protocol, it will respond with an error (such as 426 upgrade required) that includes in its headers a sec-websocket-version header with a comma-separated list of the supported protocol versions.
...And 2 more matches

Writing WebSocket servers - Web APIs
the client will send a pretty standard http request with headers that looks like this (the http version must be 1.1 or greater, and the method must be get): get /chat http/1.1 host: example.com:8000 upgrade: websocket connection: upgrade sec-websocket-key: dghlihnhbxbszsbub25jzq== sec-websocket-version: 13 the client can solicit extensions and/or subprotocols here; see miscellaneous for details.
...if the server doesn't understand that version of websockets, it should send a sec-websocket-version header back that contains the version(s) it does understand.

HTTP headers - HTTP
sec-websocket-version ...