Ok, so what you have is a 1988 model year "C" model Yamaha built Mariner (built for/sold by Merc).
(there was a 40A, 40B and 40C (first run and twin carb/oil injected sub-models) model Mariner - yours is the "C" it's just not on the little plate and since you have an electric oil injected model 40C EO is correct)
It is mechanically a 1981 "F" model Yamaha outboard - just painted different with the Mariner decals.
Your model/serial number breaks down as:
40 - horse power
E - factory electric start model
O - oil injected with the "auto blend" system which was a remote oil tank that was fed from the gas tank and then a hose from the auto blend went up to the fuel line connection on the motor. Most of these failed and killed motors so they were often tossed and mixed gas was used.
6E9 - designation meaning it's mechanically the Yamaha model 40F
S - short shaft - so for a 15" transom
105955 - the actual serial number indicating a 1988 production.
This model was widely popular. Parts are still very much available but not "all" parts. Production stopped well over 20 years ago and it was older technology even when they were built. Yamaha stopped making the model for themselves in 1983.
Here's a link to this websites parts availability for your model:
http://www.marineengine.com/parts/m...-carb-wautoblend/6e9105014-thru-6e9150000---e