The parties or either party to a dispute between an employee or a group of employees and a carrier or carriers by air may invoke the services of the National Mediation Board and the jurisdiction of said Mediation Board is extended to any of the following cases:

((a)) A dispute concerning changes in rates of pay, rules, or working conditions not adjusted by the parties in conference.

((b)) Any other dispute not referable to an adjustment board, as hereinafter provided, and not adjusted in conference between the parties, or where conferences are refused.