From what I understand all of the Gilbert and Sullivan operettas are in public domain and are not subject to any royalty or licensing fees, but for The Pirates of Penzance MTI licences the Joe Papp/Public Theatre version which is still under copyright and subject to licensing fees. What are the differences between the version of the show in the public domain and the Public Theatre revival version?

Compared with traditional productions of the opera, Papp's Pirates featured a more swashbuckling Pirate King and Frederic, and a broader, more musical comedy style of humour. It did not significantly change the libretto, but it used an adapted orchestration and made a number of key changes and other minor changes in the score. The "Matter Patter" trio from Ruddigore and "Sorry her lot" from H.M.S. Pinafore were interpolated into the show. The production also restored Gilbert and Sullivan's original New York ending, with a reprise of the Major-General's song in the Act II finale.