The Merc shifts by rotating a splined shaft. There is a male stub, splinde, shaft at the front of the LU and a female splined union on the end of the shift shaft that comes down through the mid section from the shift lever adjacent to where the shift and throttle linkages are and remote cables mount. The interface at the mid section to LU is slide off, line up the splines, insert and push back on.
Corrosion can be an issue that far back (94 year model). A problem may occur if the shift shaft stub in the LU sticks to the union from above as you are separating.
If the shafts aren't worked free before the LU is fully separated, the stub shaft in the LU will be pulled out of it's socket in the shifter cam requiring a tedious reinstallation.
If the stub shaft is accidentally pulled out, reinstalling it in the cam is very difficult through the hole in the top of the LU casting due to the spring loading of the cam follower shifting mechanism pressing against it.
Removal then may/will be required of the rear prop shaft bearing carrier assy including the R gear and then the prop shaft and probably the F gear to gain access to the cam to put it back in it's retaining slot so that the splined stub can be reinserted into it through the hole in the casting from above.
In short, make sure it separates as soon as you get enough space between the LU and midsection to get at it. Then you can avoid all that gobbledegoop.
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Since you wanted to know how the linkage works, the actual shifting is the reason for the linkage so the shifting is accomplished:
The drive shaft has a pinion on the bottom end that engages F and R gears, turning them as long as the crankshaft in the engine proper is turning. The "clutch dog" is the gear selector device and rides on the prop shaft. In N it touches neither F or R so the prop shaft is stopped, the cam follower on the front of the prop shaft is in the center hole in the cam that is positioned by the splined shifter stub mentioned above.
If shifted into F gear, the cam has the shortest of the 3 detents and the cam follower moves forward forcing the clutch dog to mesh with F gear and the prop turns CW. If shifted in R gear the cam rotates the opposite direction and the longest detent pushes the cam follower the max distance to the rear and the clutch dog engages R gear turning the prop CCW. That's it.
