Mystery Piano Music Notation

Not a standard symbol I've ever seen, but from the context, I'd the most likely explanation is that some editor invented it as a suggestion to the player that the two notes in question can be played together with the thumb.

I have seen this on a score a while back, not sure exactly what it is or what it is supposed to do, but I used the square arpeggio bracket from the apreggio palette to enter it visually.
It didn't sound any different with our without it.

Then again, it could just indicate an arpeggio starting on that note and going up.
The engraver may have just been lazy by not bracketing the whole chord.

My guess is that it's a fingering indication play the lowest two notes of the chord with the thumb.

It is a guess though :)

Definitely a non-standard marking.

...the piano is a 'string' instrument. ;-)

Therefore, to make a comparison, any guitar player should instantly recognize notation like this:
NOT FOUND: 1

Also, for bowed instruments, page 42 of Gerou and Lusk's 'Essential Dictionary of Music Notation' shows:
NOT FOUND: 2

So, from the Mystery-Notation.jpg (and actually trying to finger that chord on a piano keyboard), the bracket does seem like a suggestion to 'barre' the two notes with the thumb.

Since the bracket symbol is used twice in the piece, it would be interesting to know if its second use is on the identical chord; or with a different chord requiring the same kind of fingering.

Regards.