I'd say it may forever be a mystery
might have been something that was only used on the step vans, or maybe a rare aftermarket device [from the van body mfgr?] that was an attempt to prevent the box mechanism from jamming the way they tend to do, looks like it would limit travel of the forks - did you investigate how it would affect movement when in place? ..... that shift box has the style arms used in 55-59 trucks, not the earlier years
Bill
