Search:

Come to think of it.
If it's a vintage Coticule, glued to its back, there's a high chance that some of these methods will dissolve the glue, that was often based on beeswax. Be prepared that you...

I have no other advice to offer, other than what's already been mentioned.
If nothing works, you could also decide that you don't mind. Add a bit of detergent to your honing water and use the hone...