Purchase the correct size replacement: https://www.westmarine.com/suncor-stainless-steel-bow-and-stern-eyes-P002_060_008_504.html Then remove the old one by unbolting it or cutting it off and using a punch and a hammer to push it through the transom. When installing the new one put some 4200 sealant on the threads where it will be inside the transom and torque the nuts down nice and snug. Hardest part will be the contortionist act to get to the nuts on the inside of the transom in the engine bay.
Port/STERN. Gotta say - I have never seen that - and I've been in this business for 35 years. That's a first for me! As noted, it just unbolts from inside. It appears there may have been some crevice corrosion setting in - this happens when items aren't re-sealed on a maintenance level every so often. Water (a small amount) stays in contact with the SS and prevents it from access to the air - without access to air, SS will rust. Make sure you get a quality replacement.
To clarify it is the port/stern. The port/stern buckle will not release. I'm going to cut it off. My concern is will the boat shift on me?
Filiform corrosion. We see that in the semiconductor equipment realm. You can section a piece of 316L across the tiniest little pinhole, and the insides look like a cross section of an ant's nest. However, zooming in on the picture makes me think that rust we see may be more related to the s-hook on the end of the tiedown strap. Do you have far to tow? You could tie down to one of the gunwale cleats and take it easy on the trip back. YMMV
