It seems as though editions 12, 14, and 16 were never Incorporated. Understand that not all editions of the API 1104 were Incorporated by DOT. If it is your choice or if your requirements are that you use the latest edition there is no problem with that as long as you satisfy all the requirements of the current Incorporated By Reference edition. As long as you are using the most strict requirements from either standard you should be safe. Sometimes state regulators may lag behind the Feds a little bit, but keep in mind that the Feds always trump the State. And as you mentioned your State regulations may also list of approved codes and standards. Review relevant DOT parts and the API 1104 to see which Sections are referenced and within the 1104 which Sections may be referenced.

Not all of API 1104 is Incorporated By Reference. Continuing to use an edition that has been replaced by a more current edition is not typically frowned upon as long as any additional work is in compliance with the current Incorporated edition. Currently as of Januthe 20th edition of API 1104 is still the listed edition therefore it is required that this edition be used for any work that is covered by DOT. DOT CFR Part 192 and 195 has lists of documents that have been Incorporated By Reference (be sure to read the explanation of what Incorporated By Reference means).

Api 1104 code book 20th edition pdf