One quibble: As a Master and Commander, Aubrey would not have been entitled to 2 epaulettes. The sequence was: Master and Commander, 1 epaulette, left shoulder; Captain of less than 3 years seniority, 1 epaulette, right shoulder; Captain of more than 3 years seniority 2 epaulettes.

Besides which, a master and commander would not have been appointed to command a frigate, which was designated as a rated vessel (6th rate). Those were for captains. In the event of a really successful action, the first lieutenant of the vessels involved might have been elevated directly to Captain. This was considered a mark of approval of the captain's behavior.

In the first book of the series, Jack Aubrey, an unemployed Lieutenant, is appointed Master and Commander of the Sloop Sophie. The next book has him rated Captain. Especially in foreign stations, appointment as master and commander was a sign of preferment and generally resulted in elevation to captain within a year or two.