Hi, the R-35 is from 1930, just after RCA bought Victor. It's one of several models from 1929-30 that used essentially the same chassis, and they are great performers once brought back to life. You may be able to get a volume control from Playthings of Past:
http://www.oldradioparts.com/
however there's always the possibility that an old original control could be scratchy.
Or possibly Mark Oppat can make one up for you out of new old stock parts:
http://www.oldradioparts.net/controls.html