Re: P35 and P40 Lower units.

[jerry-ahrens]

The Johnson Brothers were the first to use what we know today as a Trim Tab. It mounts below the anti ventilation plate, toward the rear of the gearcase,(cavitation plate) and is held in place by two screws. You can make one out of aluminum. I’ll have to take a pic of mine sometime. If I remember correctly, it was called a ”stabilizer fin”.
Your lower unit has the cast in mounting bracket with two holes in it where the fin is mounted.