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I’ve got a couple mercs of that vintage, and I was wandering through the internet trying to find interchange info and couldn’t. I got given a frozen 7.5HP from the 80s or so, I pulled it apart and checked everything I could on the lower unit, and other than the splines on the prop hub being different, and there not being an indent for an o-ring on the top of the driveshaft I couldn’t find any differences. I slid it home and screwed in one of the two retaining bolts, then turned it over by hand and it spun.
Based on that, I’d guess that there is a very wide range of interchangeable lower units for those, but I never found anything concrete, and you might find different results based on your experiments.