What models had a spring hooked to that tab? I had two theories, First was to protect the lip of the torque tube as it was separated from the transmission as it's tendency is to drop down and in a hurry, and Second (and I felt was my better theory), tied in with the mystery key shaped hole un the cab floor under the seat. I envisioned some sort of lock to hold the driveshaft up when there was no engine or transmission to keep the truck frame mobile. (whether for service or actually in the assembly line process)