skunkedagain

Chassis grounding can be useful to reduce interference between different devices. It serves to bring the "potential" of the different devices to a common voltage. Yes, it would normally be brought to a ground plate (like a sintered copper Dynaplate®) You can use any screw connected to the chassis to do this -- like the antenna SO-239 mounting screws. Icom probably decided it had little value, and decided to stop threading the hole at some time. Just as well -- just another place where salt and water could come in, anyway.

AP was the finest course in the USPS catalog (is it still given?). I started with the old basic boating course in 1970 or so, then seamanship, and AP. Dropped out of JN. Taught Basic Boating and also Marine Electronics for several squadrons. Still have my old courseplotter, too.

"dead shorts" are common across the center pin and shell on marine antennas. It's part of the matching system (presents a 50 ohm load to the radio) that's a part of the antenna. It doesn't prove anything. A VHF SWR meter (cheap) or an antenna analyzer (expensive) is the only way to check an antenna.