It means the engine is an 'LB7', the first production run from 2001 through mid 2004. This is a very good engine, free from lots of emissions stuff added in later years.
However, the LB7 used a 'through the valve cover' design for injectors, that adds alot of time and effort for replacing injectors, thus the higher repair bills. Add to that, the
LB7s are known for short injector life. I have replaced four sets on three trucks, and the average mileage was under 150K miles.
Next assignment: Take the VIN of the Duramax to the closest
Medium Duty service center, have them 1) run the warranty history, 2) see if you still have original injectors, and 3) inquire if this GMC cab/chassis is still under the 200K, 7yr injector replacement warranty.
You'll have to do your own research elsewhere on the Ford. My thought is if the Duramax has no warranty coverage and still has the original injectors...you might be money ahead to buy the 7.3L powered shuttle, if they both have the features you need.