"Nissan is really Tohatsu - Ni
"Nissan is really Tohatsu - Nissan doesn't build outboards at all.
Merc and Tohatsu have been involved in a joint venture/reasearch together for quite a few years now.
They first built the (made in Japan) single cylinder 4 and 5 horse 2 stroke models a number of years back and then moved on to build all the portable 4 stroke models (so if you look at a 15 horse or under 4 stroke whether Tohatsu, Nissan or Merc with the hood off you can not tell the difference).
And yes, alot of Mercs traditionally never had cylinder heads (referred to as "headless" or "split block" design).
That was a sign that the powerhead had actually been built by Yamaha (who to my knowledge never used the headless design on any of their own branded motors).
So to answer your finally question "are some tohatsu really mercury engines?"
The answer is NO - BUT alot of Merc's are really Tohatsu's - Merc is not building them - they are doing nothing but providing the paint colour and the decals, the motors (whether Merc, Tohatsu or Nissan) are pure Tohatsu.
The day is coming soon when the only true Merc build outboards on the market will be the Optimax and Verado lines - and it's probably only a matter of time until production of those is farmed out somewhere as well...."
A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)