no one really knows exactly when GM started doing the NAPCO conversions themselves, late 50's sometime, the earlier ones [54+] were done at a 3rd party shop, sometimes a GM dealer .... there is no "slot" above the floor shift in any TF model, the 4x4 lever was to the right of the shifter,
like so, no tellin over 50 years who modified yours, might have been for a winch control or a dump bed
