In the good old days, many light aircraft had naught but the OEM's guidance - some had naught, full stop, with anything of vital interest being specified in the CofA.
As things evolved, the OEM data may well have been styled in various ways, eg, "Information Manual", "Owner's Manual", and so on - depended on the OEM's design organisation's idiocyncrasies. Some of these documents were, to say the least, lacking in detail and usefulness. Indeed, in Australia, the local NAA (then the Federal Department of Civil Aviation) issued its own generic Flight Manuals for light aircraft (Civil Mk I for singles and Civil Mk II for light twins, if the memory serves me). These weren't brilliant but, for the basic stuff one needed, they were a lot more useful than much of what came from the OEMs. As an Industry engineering consultant, I produced a number of these over a period of years. The advent of the GAMA format manual was much needed relief ....
The Design Standards, at some stage (I'll leave it to those enthusiastic enough to go search the FAA history to locate just when) required that some data be provided in the nature of a Flight Manual. Post the introduction of the GAMA Spec 1 manual, the OEMs progressively moved over to that more standardised format (at about the same time, ICAO put forth a similar document - Doc 9516 - memory is not adequate to recall which preceded what at this distance in time but, as I recall, there was not all that much in the datal differences). The POH co-opted the Flight Manual data and the relevant pages of the latter are readily identifiable.
The particular "Flight Manual" document, for any given jurisdiction would be controlled and approved by the relevant local NAA. It should not be surprising, at all, to see much the same aircraft in Country A issued with a Flight Manual somewhat different to that issued in Country B.
These days, no doubt due to litigation wariness, the OEM Flight Manuals/POH have gone from a sensible short document to literary works of art, extending on to 100s of pages.
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.
One of concerns in Australia was just this and that, in the main, was the reason that DCA introduced the local generic manuals.