What kind of finishes were used on guitars from the 30's?

[Speedy77]

I'm restoring an old Harmony made guitar from the 30's and was hoping to get a little help with figuring out what the original finish was. I'm fixing some serious cracks in the wood, and wanted to use the same finish that's already on the guitar. I was assuming that it's nitrocellulose, but just wanted to see if you guys had any other opinions to what it may be. Hopefully someone does! Thanks!