I just got the nifty purple-glowing skin for the cog hammer; so, of course, I immediately go to try it on. After a minute or two of examining the illusion, however, I decide, “Meh, I actually don’t like the way it takes a chunk out of the axe blade.” No problem; I can just apply a new illusion to the weapon to make it look like it did before.
Except … I can’t. There is no illusion option for the default cog hammer.
The blacksmith’s cog hammer has a solid hammer-head and a solid axe-blade. The only non-purple-glowing illusion in my inventory has two nodules on the hammer-head and a chunk taken out of the axe blade (it looks exactly like a non-glowing version of the purple skin, in fact).
It seems like we have a missing skin. At present, the only way to get the default cog-hammer appearance again is to make an entirely new cog-hammer. I suspect this was an oversight.