"There are also multiple letter codes for larger trucks indicating a COE, tandem axle, or diesel engine. GMC didn't really look at truck "years" so much as at did model runs which in many cases ran for multiple years."

see here, and for the HC451 see here - the letters should indicate a 50-51 truck, a 48 would be FF451, print out the list and take it to your officials

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics