The first two digits represent build date month and year. In this case, J is September, 2 is 1992, and 5 on the 4126 indicates it was built in 1995, as validated by the '95' casting mark.DB is for rotary distributor-type head, where each cylinder gets it's own injection port and pipe, supplied by single barrel.2 indicates two opposed pumping plungers, driven by steel rollers in ceramic pillow blocks8 indicates number of cylinders31 indicates 0.31" pump plunger diameter, used in the 6.5 pumps. 29 is 0.29" used in 6.2 pumps. 4911 indicates specifications to which pump was built, J is assembly plantAlso on label is inj pump serial niumber, and customer part number - GMC, Chev, Ford, etc