I believe those are the default icons that ship with UBB.threads 7.x. IIRC, the decision was made, back when MFIF's software was upgraded from v6 to v7, to replace these stock images with the ones we were then using, which were found to be much more legible, due to larger size and simpler detail—and thus more likely to be useful at conveying extraverbal meaning—than the new roster.
Similar reasoning applied to the smilies used within posts, and at the inception of FTM, I think we felt that the familiar post icons and smilies would help contribute to the sense of continuity with MFIF that we were striving for, so the replacement of the newer sets of images with the older sets was one of the many tasks accomplished during the 24 hours preceding FTM's launch.
I note now, though, typing in Dark mode, that no smilies are available to be used within the body of my post; they're all question marks (in Safari, that is). Oddly, the replacement of the default posting icons in the default skin (ubbthreads_finetunedmac)—which translates fine into the "stock" skin (ubbthreads_stock) as well—seems to have had no effect on the icons available in ubbthreads-dark; whereas the replacement of the default smilies (which also translates into the ubbthreads_stock skin) has displaced the default smilies in ubbthreads-dark.
It's as if each skin maintains its own folder of images, and thus each skin is instructed to follow a different path to find a given image, and in fact, research bears this out: in ubbthreads-dark, post icons are kept in /forums/images/icons/default_dark/ and smilies are kept in /forums/images/graemlins/default_dark/.
Why replacing the default post icons had a different effect in ubbthreads-dark than replacing the smilies did isn't immediately clear, but I guess if we're trying to maintain compatibility between skins, we'll need to address the issue at some point.
