In most OMC cases, the combination of the HP and the year model is what is needed to lookup OMC engines using the service literature provided by OMC to their dealers. In some cases, both the year model and HP are part of the model number. In other cases, a lookup table can be used to convert to what is needed. For this reason, you should always get the model and serial number off the manufacturer's plate attached to your engine before attempting to buy parts for your engine. The red circle stickers are reflectors that were added after the purchase of the engine.
Engine companies change motor cover graphics designs periodically.
For OMC, this is almost always a year model matchup. All HP engines of a certain engine had similar hood graphics. 1977 was the first year for white on white Johnson engines. For some other engine brands, it is not as scientific. The goal at this point is to get one side view picture for each year of each brand of engine for the past 40 years.