The only articles that I write about are ones that I personally have worked on that motor, so you may see gaps in my articles.  However I have had a lot of requests for information on these older small motors, therefore this is info that I have found on OMC parts lists is as follows.

A lot of parts interchange between models and HP of closely associated motors in those days as it was simple to create "Families" of near HP size motors using the same parts.

The early smallish motors (other than articles I have written) are the 5.5hp which were made from 1955 to 1964. 

In 1965 the 5.5hp was dropped & replaced by the 6hp which was made up to 1979. 

Then from 1980 to 1983 they dropped the 6hp & brought out the 4.5 & 7.5hp (both on the same pattern) which was made up to 1983.  

In late 1984 they dropped the 7,5hp & went to 6hp AND an 8hp.  This model change (upgrade in a lot of things including the rope starter) was made in 1985 which went until 1991.  Basically this would have been some improvements, but mostly a different carburetor.  

The photo below shows the a multi -function idle/kill button on the end of the twist knob.  This style handle is activated by a internal cable from the handle to the carburetor.  Inside the outer rotating knob, which is a low speed adjustment, is also a push button kill button.  This rotating button/end knob when used for an low speed adjustment does not have any carburetor adjustment, but is used mostly by trolling fishermen who want a preset low speed other than having to each time they twist the handle to any other position, and want to go back to their preset slow position.  In essence this is just a fine tuned low speed setting.  The knob can be turned in or out.  Out will usually come to a stop after a few turns, where in, once it has passed the few threads on the large inner section of this knob, will just keep rotating, but do nothing because it has simply ran out of threads.  If you can not get it to adjust the cable to the linkage to the timing plate by unbolting it there and screwing the outer plastic either in or out if you want to readjust the indicator pointer to the speed letters.  This can become frustrating.

 

To disassemble, push in on the 2 plungers simultaneously  (top & bottom) & pull the twist handle forward.  This reveals the slider cams which engage a cross-pin which activates the in/out cable movement. 

 

In 1992 they brought out a totally different design, lots of plastic & shifter lever on tiller handle, etc. this was made until the end of OMC in about 2005.

I have had numerous inquiries about a simple cheap upgrade from the 6 to 8hp motors indicating the removal of a restrictor plate behind the carburetor.   Well in Europe, it was common to downgrade a larger motor, the information I have, this would have been on the 15hp made from 1974 until probably 1992.  However the only motors I have heard of doing this, used a restrictor plate between the carburetor & the manifold was the 15hp, economically dropping it to about 10hp to comply with local HP restrictions.