I picked up a Marinetics A/C breaker panel that has main breakers and
5 branch breakers. It seems the company is no longer in business.

snip

The questions I have a

Does anyone know why the spare breaker isn't connected to the Line
jumper?

Someone disconnected it?

Another question. There is a "on/ok" green pilot light, with a
"polarity test" momentary switch below it.

I havent installed the panel yet. Is the "on/ok" light supposed to be
on when the main breakers are on? What is supposed to happen if the
polarity is reversed? When are you supposed to push the "polarity
test" button, and what should happen if all is ok or not ok?

The old a/c system on the boat has an old breaker box and a polarity
alarm that goes off if polarity is reversed. Is this panel designed to
be used with an alarm?

My sailboat has a Marinetics EP2635 AC panel that sounds not
dissimilar to the one you describe.

I do a polarity test after connecting to shore power in a new-to-me
marina (I also follow Nigel Calder's recommended tests, for which see
http://www.proboat-digital.com/proboat/200604/, if I plan to be the
marina for more than a day).

Pressing the test button lights the green test lamp. That's the only
time the green lamp illuminates. If the green test lamp doesn't glow,
the polarity is reversed, the lamp bulb is blown etc. No alarm for
reversed polarity.

Cheers

Bil