Most of the factory photos from Service Publications show the wing nuts in each front corner like Bob's photo in the post above but I found a photo of a restored truck in my photo collection that had a clip type catch in the middle front which would match those holes.
Hope this helps
__________________
Cheers
Cliff Hutchings
aka MrRoo S.I.R.
"and on the 8th day he made trucks so that man, made on the 7th day, had shelter when woman threw him out for the night"
MrRoo says "TRUCKS ROOLE"