Hat channel with "notch" question

[Reamer]

Im replacing the hat channels, what is the notch in this channel for? this channel is located on the right side, and is the first channel after the bulkhead wall. You can see the original rotted one to the left of my almost-finished reproduction.

[Patrick Tipton]

The early Weasels shipped with a removable "bulkhead" that would allow you to store cargo along the passenger side.  This is a receptacle for a pin on this bulkhead.

[Reamer]

Thanks.