I had my doubts like the other guys but looked anyway. Parts book for both unstyled and styled D does not show a duel muffler option.  Nor is it mentioned in any of the info put out by JR Hobbs.  About the only rare muffler for a D would be the ones that were on the right side.  In 1937 JD equipped a few Ds with a special hot manifold that included a muffler on the right side.  Most were shipped to Canada and are suppose to be rare.  Your friend got taken and I doubt the 6 hp claim as well.  If you wanted more hp out of a D, a cold manifold and gas pistons would give you about 10 more hp.