If the balancer doesn't have a mandrel to hold the coupler, it must be balanced with the flywheel. Most high performance balancing companies will balance each individual component. Then if needed they will assemble the pieces and re-check their work to make sure it's still perfect. I'll bet he didn't have the mandrel for the coupler so he attached it to the flywheel and balanced them as a pair.

The real deal is that the motor and even the coupler are made to certain production tolerances to be in a ballpark acceptable tolerance limit. This is just how it was done.
Of course a really good machinist will be able to correct a balancer or flywheel and make it close to perfect.
When bulding a H/P marine engine, this is definitely an area that pays off. When trying to make more power out of anything , the margin of error closes up, and things have to be more perfect and less forgiving... BBB

better stick the drive on too ....get it all done .....

I'm building 496's out of MIV blocks and the rotaing assemblies are internal balanced and we just took the MIV "externally" balanced flywheels and made them neutral ...and used neutral HB's .....

My understanding is, ONLY the harmonic balancer is "externally balanced" on a MkIV and the flyweel is neutral. Correct me if I'm wrong.

On the face of the flywheels there weights ( actually thicker areas) in stratigec area's around the flywheel ....The machine shop removed those and actually removed more material where needed. I know you can get them either way ....It was a lot cheaper just to get them Neutralized ...