Don't know perzactly what you've got, but I ruined an old celluloid tort guard off an epi archtop bass with denatured alcohol. That said, I've got a newish MIM J bass with an aftermarket MOTS guard on it and I wipe it down with denatured from time to time for cleaning so there is definitely a big difference in what they are made of today versus way back when.
Rubbing alcohol is not near as strong but unless you can get a replacement for the MOTS you are talking about, I'd proceed with much caution with any kind of alcohol.
"Strictly speaking, there is no certainty, there are only people who are certain." -anon-