Funny thing, a guy posted a photo on the WW2 Dodge forum of a original M24a2 mount and the corner pieces used 5 screws on each piece. Three on the side, and 2 on the front. Here's the photo, lower right.....I can't help with the locations of the holes but can with the screw sizes. I pulled my original ORD 9 on the M24A1 mount and it listed the following sizes of screws appropriate to these brackets....
The 4 screws mounted to the sides of the truck bed (2 per corner bracket) are 3/8-24 X 7/8 hex head cap screws with steel safety nuts. Didn't look up the definitions but assume the nuts are the old slit type lock nuts.
The 4 screws attaching the corner braces to the upper cross member of the pedestal assembly are 1/2-13 X 1 1/4 fl-hd screws, also with steel safety nuts. I assume flat head screws and they appear to mount from the front to the rear with the nut on the bed side. Does your mount cross piece have countersunk 1/2" holes that would indicate this? If the holes are present in the cross piece, at least the hole locations on that side of the corner brackets should be obvious. Hope that helps a bit.