If you think it is a genuine type 1 of Joe D I'd suggest investing in having a qualified conservator remove the mask. I've removed that sort of stuff very successfully on photos:
Kid Kaplan before:
Kid Kaplan after:
But there is still a residue. Yours looks like it would come off really clean, since there is no apparent india ink or white paint, just the gray stuff, which is the easiest to remove in my experience.
I wouldn't trust myself on a potentially super valuable item.