Since you didn't mention the horsepower... In 69 there were two types of "Selectric" shifting systems. One was the Electric system used on the 40hp, the other was the Hydro-Electric System, used on the 55, 85, and 115. The Hydro-Electric system defaults to forward gear and if no power is applied to a particular coil/solenoid to shift into Neutral, or the oil passages are plugged, or the oil pump has failed, the unit will remain in forward with the prop spinning. The older Electric system very rarely locked up in gear, it usually broke resulting in at least two neutrals.
There is also some residual friction which often does result in the prop turning at a much lower speed than if it were in gear. Do not touch a spinning prop or try to stop it.