I've recently acquired a Model 1868 with the matching serial number 20251 on the left side of the receiver and barrel as one would expect and no year on the breech block. It has what is probably militia stamping on the top of the stock behind the hammer/receiver, but it also has the number 6008 stamped on the right side of the barrel just in front of the receiver. I've seen a couple of references to such markings, which appear to be stamped as a group, not as individual numbers, but does anyone know what they mean? Are they another form of downstream user markings? Thanks.