THIS cap is only 3 inches tall the hump near the cab is 6 inches it has a trap door that locks im going to take a pic and post it in 79 when i bought my truck new the dealer had one on the floor in the accessory section it was quite pricy so i opted out i worked at gm powertrain at the time and saw at 3 on trucks in the lotwith them i just found one in a local junk yard im putting it on my shortbed 86 as soon as i paint it tan now my wife wants one on her 76 gmc