Any logical reason for the previous owner to have added a bolt to the bottom of the gas tank?

[kiwigtx]

Don't be silly. The only other possibility is a negative ground post for the trunk mounted battery.

That would mean there must be a cable-entry hole in the rear of the trunk compartment for the wire to go through???

Or is it a quick-release like hood pins?

 

[WileE]

That would mean there must be a cable-entry hole in the rear of the trunk compartment for the wire to go through???

Or is it a quick-release like hood pins?

Sorry, I should be more specific. It's a negative ground post for the under-trunk mounted battery.

 

[kiwigtx]

Sorry, I should be more specific. It's a negative ground post for the under-trunk mounted battery.

Oh...that would make perfect sense then.

 

[WileE]

Oh...that would make perfect sense then.

It does ?? { psst... if I make sense to you, you've entered an alternate reality, my friend }

 

[khryslerkid]

The area around the 'bolt' has been cleaned with a wheel or grinding disc. That looks like a purposeful bung like in your engine sump.
Someone did this on purpose as the bung is in the lowest point of the tank for drainage.

Leave it alone.

Looks like JB Weld and some orange silicone.

 

[wwheel]

Mercedes of the 50s, 60s and 70s all had tank drains from the factory. Maybe other Euro makes did too, not sure. Damn handy.

 

[EmitRDetsaw]

Nothing is more permanent than a temporary fix that works.

 

[OKDart]

Mercedes of the 50s, 60s and 70s all had tank drains from the factory. Maybe other Euro makes did too, not sure. Damn handy.

View attachment 1511841

Mopar had them up to at least the 60's too, not sure when they stopped, but I have a 57 & 58 with them, also a '66 Imperial. I agree, very handy, especially for a car that doesn't get driven a lot. I can vouch that they last for at least 66 years without leaking if left alone.
I think the one in the pic below has been modified with a gasket, but original would be more of an o-ring. Also the plug is made so that it tightens flush so there is also somewhat of a metal to metal seal.

 

[detmatt1]

I have a ‘64 Imperial with one.

 

[YY1]

My 73 came with a nickel sized dollop of JB Weld in that location.

It lasted about 3 months, then it was "new tank time".