As I'm back in the office, I'm able to refresh my memory about the Piper Twin Comanche "Flight Manual" from my paper copies:
For the Twin Comanche I flew, I have a:
"Twin Comanche C PA-30 Owner's Handbook - Piper", which is about 80 pages. On the second page "Notice" it says in part: "The handbook is designed: ..... 3. to more fully explain your Twin Comanche's operation than is permissible to set forth in the airplane flight manual. If there is any inconsistency between this handbook and the airplane flight manual approved by the FAA, the airplane flight manual shall govern."
I did observe during some performance testing that I did on this particular Twin Comanche, that the some of the performance tables offered in this document (which are not FAA approved, and not required to be provided at all, were very optimistic. I think that the marketing department had input.
Then there is (in a much less official format) the:
"Airplane Flight Manual for the Piper Twin Comanche Model PA-30"
"Report 1515", which is made out for this serial number aircraft, and FAA approved by J.W. McNary February 5, 1963. This predates the GAMA format. It contains ten pages of information, the minimum requirements applicable to the aircraft, and then comprehensive W&B information for this serial number aircraft.
The pilot is bound by the "flight manual" not the "Owner's Handbook" - if they are not the same document.
Interestingly, I had a look at the Piper Cherokee 140 B "Owner's Handbook I have, and it has the similar passage on the second page, directing the reader to an FAA approved flight manual for that aircraft (which I do not have).
I was warned by a mentor to be cautious with Pipers, that you were using the correct document. That same mentor told me (which I see for myself) that Cessna always put everything in only one document, and identified its status correctly in the fine print.