Look down under the cowling somewhere on the side, on the Mid section. Should be a QD number there. If its a 56, its a QD17...I don't have a serial number guide
Metal hood or fibreglass? If metal, what color? Single or dual line fuel line?
From that we can get really close. Or go here and look at pics and compare. Or post a photo.
old-omc.de/
Oh, and a '56 10 has a little circle and a 10 in white on the side. like this: