No answer from the "General Boating" section. Bumping it to the Q&A forum:

What years did GIL manufacture and sell "GIL Magnum Choice" exhaust manifolds?

...and what about "GIL Offshore"?

Thanks.

The manifolds were all basically the same. The risers were all that was different.
Gill sold the same stuff, with only minor changes for years/decades.