According to the 38-39 book the same pair of exhaust manifolds were used 78-9439/9431, these may be the same for the commercial (trucks). There were some "experimental" 91A engines and it is currently unknown what those 500 or so were like, more Ford folklore.
Pictures would be helpful, also what is the flathead. My 39 had a 59AB and have those manifolds taking up space somewere. I resurrected my truck with Reds headers and those sweet sounding Smithy's
Not sure were you are located but the spring swap meets you may want to try. If I locate my old manifolds and they're not what you already have, you can have them, just pay for shipping. Scott