RHGI would translate to the 8592nd gun produced under the all letter S/N system. It's based on F=1, I=2, T=3, C=4 etc... From the name FITCHBURG, with X=0 .Welcome to TFF.
The RHGI is the serial "number"
Champion's with all-letter serials are the late ones made ca. 1940-1956, but I can't help with value.