It took me a while to find an image (I see them and then forget where they are!). As far as the inside tub area is, where the front seats rest back on the wheel wells, I only know of the hat channels that reinforce the attachment between the side panel and the wheel well. For reference, in this image you can see the small bracket that the seat rests on on the front of the wheel well (the driver's seat has a bolt inserted from underneath into the seat frame at this location on the driver's side).
From the CJ3B Page Seat article, Early Seats section:

I'm wondering if that triangle brace in your tub could've been added by a Previous Owner (PO) or have a purpose I'm not aware of.