Hopefully this will answer your questions.

We found that many customers do not like to countersink drill, specially their outboard's cavitation plate. Instead of using the supplied flat head bolts they used pan head bolts.

We have been testing Permatrims using pan head bolts and have found no cavitation or other water flow problems. And that pan head bolts make it easier for the customer to install (no countersink drilling). Thus, we are in the process of switching to pan head bolts.

Your plate may have included pan head or flat head bolts. This is to inform you while the flat head bolts give a flat surface for water to flow over and may look nicer, either bolt type is okay to use - it's your choice.