For what it's worth, the old-style icons (props = torus) are stored as individual layers within Runtime:ui:27100_library.psd. The new-style icons (props = hat) are stored as separate SVG files in the Runtime:ui:svg folder. The difference being, of course, that SVGs are vector images which allows the UI to be scaled to suit modern hi-res displays, whereas the PSD layered icons have a fixed pixel size and will end up tiny if your monitor has a high dpi value. My surmise is that the PSD layers are called up first by some bit of legacy code, and then are usually overwritten by the SVG versions - except that sometimes the library browser misses that bit of information.