You are on target with the 'serial number range' issue. I've been doing research on the 1st California Cavalry in Arizona/New Mexico/Texas during the Civil War, and have a copy of the weapons and equipment inventory for one company. Of the approximately 130 M1863 Sharps carbines listed with serial numbers, the serial numbers seperation ranges across a span of more than 4,000. A similar random pattern exists for both their M1860 Colts and their New Model Remingtons. If there were a list of Custer's LBH Springfields, it would almost certainly exhibit a similar random distribution.