Matty,
Is it pretty much confirmed that ONLY the I/O 16's had the floor vent and NOT the OB hulls? The reason I ask is if that is fact then there is no reason the picture's I posted could not have been the first experimental OB built by the factory. Someone on the board with intimate knowledge of what Mercury was offering for an OB back then would have to provide some insight but based on how busy the factory was building 16 Ski Sporter's at that timeframe, the only reason they might have experimented with possibly building an OB version of the 16 was because of a pending "new Mercury" OB being offered. If 1968 would have been the 1250, then the question for the historians is what would Merc have had in 1964/65?
Would a powerful enough eggbeater have been offered then (1965) to warrant the factory starting to prototype a 16 OB?
1972 16 OB - C16B-63 - "Surface Tension" Resto Project
1974 16 OB - DMR16106017-B - "The Mule"