[OMC-Boats] Outdrive Vintage Question

My Explorer is a 1970 model according to the plate on the riser.  In studying the service manual as I start to dismantle the outdrive, it suggests that the outdrive is a 1967 or earlier model.   The manual states;

"Remove the exhaust housing cover attaching screws.  On models since 1967, five screws secure the exhaust housing cover in place; prior to 1967, only four screws were used."

Is it normal that a '70 vintage Evinrude would have a '67 or earlier vintage outdrive or should I assume it's been replaced at some point?

Thanks.
Received on Sunday, 24 May 2009