Well there is actually quite a story to it, which I learned from The Old Outboard Book, which I will give the short version of as I hate typing
The first rowboat motor was built by Caille called a Hiawatha. They werent actually "Sea Kings" though so dont know if this is pertinent to your question but mentioned it none the less.
Then Montgomery Ward changed its marine line's name to Sea King where they started purchasing motors from OMC. The first Sea Kings were actually Lockwood Chiefs that OMC had that they just stuck a Sea King decal on, as the Lockwood line was discontinued.
In the late 30s Montgomery Ward also bought some Muncie built outboards. These "Thors" were also just relabelled with a Sea King decal. They ran like crap though and gave a bad reputation. While this is going on, Mr Kiekhaefer bought the Thor factory and fixed up some rejected Sea Kings, and earned a new contract with Montgomery Ward. So the early 1940s Sea Kings were a mix of Kiekhaefers and Gales by OMC.
In the 50s I think Gale supplied all of the motors with their private brand outboards.
Then in 1964-65 Montgomery Ward contracted West Bend to supply the motors.
Thats it in a nutshell, so the answer to your question is yeah, they were not always OMCs, nor did they end up so.