After IMAI go out of business, ERTL has made sailing ships kits from IMAI molds: Mayflower, Golden Hind, Chebec, Spanish galeon etc. IMAI has released Pirate ship too, and that ship was the same Spanish galeon, but molded in more dark brown plastic and has other sails decals than Spanish galeon (only that difference). Despite ERTL has released IMAI's Spanish galeon, maybe has released "Pirate ship" too, as IMAI did.
If You can post pictures of the kit, i can then sey exactly, which instructions (IMAI's or Revell's) You need. But i think, that IMAI's.