Alright, I am no longer very certain at all as to "backwards compatible", but in theory, this should be, and will update your game, whether v1.2 or higher, up to v1.26p. This is basically v1.24 with the 3x sil files "fixed", as well as some changes to Campaign files for a few issues in US_NavalBases, and quite a few of the _Jap_ files, for changes made to another ship. You should now see all of the Frigates and a few other ships in the Museum, which were missing previously.
Do not apply this mod in the middle of a patrol, but you should be OK in base, Saved & exited, then modded, and load the Save... maybe. I will attempt to get into testing Save files again, and see if I can figure anything out. I am slow though...
101_Patch_v1.2To_v1.26p.7z