Is it just me or might there be a 6 and an 8 on opposite sides of the "L"?
A union pin is a good idea... It would explain all the numbers, what with local #'s and district #'s. And it sure looks like it was a pin, based on the back (unlike, say, a mailbox tag that just stays in place, which originally was along the lines of what I was thinking).
I found little online about the American Longshoremen's Union, but from what I saw it was only around from 1896-1899, based in NY, while other Longshoremen unions were more prevalent. I guess what I mean is "did they have enough chapters for their pin to be so cluttered with numbers?
Maybe the American Labor Union?