The D'Oyly Carte library rented out "authentic" orchestral sets of parts (minus a full conductor's score) for years (they still do), but these were not really fully accurate due to various accretions and changes made through the years by house music directors and others. It is also worth noting that they also made available separate parts for the on-stage military band which played during the March of the Peers and the end of the Act I finale, and these werer also derived from Sullivan's original scoring.
Among the pirated and non-authentic arrangements of the score was one published by Kalmus in the United States. At some point about 1970 they obtained an "authentic" orchestration and offered it in their catalogue under the same catalogue number. For a few years they offered both the "original" and "arranged" versions, but soon they quietly dropped the "arranged" version and now sell only the "original", which comes with an optional (expensive and not terribly well executed) full score but sans optional military band parts.
Opera companies which have older copies of Kalmus parts for Iolanthe, Patience, Gondoliers and some of the other operas may be unaware that their orchestrations are arrangements rather than versions which follow, with reasonable closeness, Sullivan's original orchestrations. They may not find out until it's too late that if they purchase extra parts or replacement parts for missing ones that they are incompatible with their old set, and that their old set is not desirable anyway.
The new critical edition, in preparation by Broude Brothers Limited, is now underway and is among the earlier operas scheduled to be published. It will have a fully annotated full score and libretto, and will contain both the on-stage band version and the pit-orchestra-only versions of the two numbers mentioned above. The volume will be published in conjuction with a critical edition version of the piano-vocal score and orchestra parts.
Updated 28 November 1997