The Stovebolt Site Forums The Shop Area Making a Stovebolt Bed Cross sill question

What are the three square holes on the ends of the cross sills for? More specifically the second sill from the front. I know the round holes are for the angle strips and bed strips but the square ones have me scratching my head.

Then they've served their purpose.
They may be there for fabrication purposes (lining up the rest of the holes?)
I don't specifically recall when I assembled my bed, but aren't the holes toward the end for the bed to-frame bolts. I believe the center cross sills are identical and interchangeable. Might be a question to pose to Mar-K.

Could they be for a different type of bed, like a stake body or flat bed?

I have one that has no holes at all, it was bought for a bobber type truck and it was leftover. I need to modify it to use a 47-48 under the bed frame gas tank with a late 51 8 board box. So what I can figure is I need to notch it , weld a piece of 16ga to the notch and drill the holes for the wood.