Here in the USA, there are provisions to allow an operator to produce their own flight manuals. The requirements are stiff and require the operator to show that there is no degradation of safety. Having the ability to manufacture your own flight manuals can come in handy, particularly if you operate a type of aircraft that has various different limitations. You can standardize the limitations to the lowest common denominator. You can make your procedures fit what you are currently doing within the rest of the fleet. I have worked for several operators that used their own flight manuals. In some cases they were actually better than what the manufacturer produced.