I've inherited my grandfather's Gibson Southern Jumbo. He bought it new, we think in 1954. It has had some work done on it, (crack repairs, new bridge, new tuners), and needs some more done. I'd like to know how to decipher the serial number. It appears to be ink stamped on the neck block inside the soundhole. It's either Z1932 or possibly 71932. It has 19 frets, I've read that they went to 20 frets in 1955 but I'm not 100% sure on that.