Go your own way...
Who knew that the object of Twain's attention would be something so sought after, so controversial, that it would lead to countless internet battles.
It's vague. "object". It could be anything, right? We can only assume that was intentional. Not only is Twain one of the more prolific writers in American history, he literally left his mark (no pun intended) all over California, nevermind just San Francisco.
I'm always looking for determinism in puzzles. How can we narrow it down in a deterministic way? Fenix would suggest going definitional: what is an "object of someone's attention"? Consensus there is a job or a hobby. Before you run screaming to Aquatic Park, let's look at the painting.
Hypothesis: The vagueness of the line, will be resolved by confirmation in the painting.
There are a number of supposed verse/painting confirmations in the casques that have been found. I won't get into the weeds with those details here.
We can argue about macro clues and micro clues in the painting, but one thing that we can't (or maybe better to say -- shouldn't) argue about is that the Presidio and part of the Golden Gate bridge is in her hair line.
Yes, the Presidio tells us San Francisco, but perhaps it's telling us more. At that spot where the Presidio turns into the Golden Gate Bridge, you'll find Fort Point. At Fort Point, you'll find a plaque with a quote from a famous San Francisco news reporter.
Was it there in 1980? The jury is still out. I'm constantly scanning youtube and facebook groups to try to find anything relevant from our most coveted time period.
The conclusion that you should absolutely not draw from this, is that the casque is in the Presidio. It's not. The Presidio was an active US Army base in the 1970s-1980s. Do not even consider digging there. That's my PSA for today.
Clues often fit into puzzles in ways that we might not naturally expect. That's why they're so puzzling. This clue is directional. It boils down to something like: wherever you are, head in the direction of the object of Twain's attention to find the casque. Until we can determine the correct object, we're going in the wrong way.