1994 Mazda B3000

I have a 1994 Mazda B3000 extended cab pickup.  There are notches right
behind the front seats half way up from the floor.  I first thought it was
for a rod to hang clothes etc. but it is too low.  Is this for some sort of
platform/shade to cover contents you might have in there?  If anyone can
shed some light as to what it might be I would be much appreciated.

Thanks for all replies...
Cretemite