May posting a notice to competitors on the official notice board without displaying flag L with a sound be considered an improper action or omission of the appropriate committee, for the purposes of rule 62.1(a) while all the other requirements for posting a notice have been met?

It would depend on the circumstances.

Although flag L displayed ashore with one sound signal means, in Race Signals, that a notice to competitors has been posted, there is no rule requiring the display of flag L.

It is the responsibility of a competitor to check the official notice board, however, displaying flag L draws the attention of sailors and if the notice contains information which could affect a race result, failure to display flag L might be an improper action or omission for the purposes rule 62.1(a).

Even if the protest committee is satisfied that in the given circumstances the failure to display flag L when posting a notice to competitors is an improper action or omission of a committee, in order to give redress, the other requirements of rule 62 shall also be met.

It is good practice from race officials to draw attention to the posting of the notice, especially when the notice concerns a matter with immediate effect.