Newcomers to this page may wonder why there are so many pictures of the mailbox on top (I added one myself). One reason is that this peak is a special place for hikers, and the mailbox is a kind of "secret handshake" for those for whom hiking is more of a lifestyle than a sport.
I am waxing a bit philosophical here, but to me, the peak itself and the mailbox, indicate a familiarity, or at least a willingness to become familiar with, the special delights and miseries of North Cascades travel.
If I see a guy by the mailbox, I can be assured that he has, or he wants to tackle rough n' ready trails around Glacier Peak or Monte Cristo. I'll bet he's heard of the Ptarmigan Traverse. He probably knows where Sloan Peak is.
The mailbox is an unspoken joke and bond between a pretty small group of like-minded folks. And inside jokes or references are always fun!
Of course, maybe the fire training camp below spoiled it by bringing an ungodly amount of things up later, like a ladder and a fire hydrant. :-).