Astro VIN TDH92AV
T= GMC, D=Aluminum Tilt Cab - Tandem Axle (Astro 9500), H=Detroit Diesel 8V-71, 9=GVW (9,000 series), 2=Brake/Body configuration, A=year of production, V=Built in Pontiac, MI
7500 Conventional VIN TJV73DV
T=GMC, J=92" BBC Conventional Cab - Tandem Axle, V=Detroit Diesel 6V-53, 7=GVW (7,000 series), 3=Brake/Body configuration, D=year of production, V=Built in Pontiac, MI
The T/C prefix (GMC/Chevrolet) began with the 1972 production year so both trucks are 1972 or newer. I haven't found confirmation of the Letter=Year designation, but until we can find better documentation it would be safe to assume that the astro is a 1980 model and the conventional is a 1973 model. I also haven't been able to verify the updated Body/Brake designations, but most likely the Astro having a "2" would indicate a Tractor/Air Brake arrangement and the 7500 a Cab Chassis/Air Brake set up due to it's "3" designation.
I am still trying to confirm this information in the library so if anyone can add to this, please do.
I hope this will help,