Hello I have a 1953 Chevrolet 1300 - the Canadian version of the 3100 (or so I am told)
I have been working to get my truck on the road. I discovered what looks like a factory hole in the floor pan under the seat.
Ok - why is it there? What use does it serve? Is there supposed to be a plug that goes in it? Sorry for all of the dumb questions.
I am still a newbie... and not sure who locally has an old beauty 1300/31oo to ask. Any help would be appreciated in identifying this?
The picture attached shows the slot.

Are you referring to the keyhole shaped slot? If so I have no clue. The only hole under the seat on my '51 is a bolt hole for a wiring clip toward the driver's side. There's also a 1/2" or so drain hole in the center of the depressed area under the seat.

Maybe a "drain hole"?
Here is a "plug". I do not know if it is original style.

Jim Carter sells a few sizes.

I think I had a rubber-like plug in the hole in the floor under my 54 Suburban's 2-person seat.
I dimly recall a vendor selling a replacement rubber-plug? - I think I am dreaming?

Chevs of the 40s - a sizes of metal plugs.

If they were originally metal, they probably rattled free; and, or, they rusted away?

Can't enlarge the pic to see which hole the OP is referring to.

On my Macbook laptop keyboard (macOS using Chrome or Safari browser):
- click on the photo/image (to display the image at "natural size)
- move the cursor/pointer over the photo/image
- press (at the same time) the command key and plus key (multiple times for more and more enlargement)

I am using a keyboard.
I have no idea how to do that on a tablet/smart-phone.

Post the device, and the OS, and the web-browser software you are using.
There would be different strokes for different folks/devices.

I have the same thing. The shape of the hole suggests that a bolt passes through the center and slides right or left for a hold down of some sort. Maybe it was used to hold the jack in place?

That figure from the FAM the OP posted looks like some sort of cover over the seat riser, and not the floor pan. But maybe that's just an optical illusion or a drafter that didn't draw it correctly. The figure on the next page looks more like what I have, but it's for the big bolts. Again, I don't have a hole shaped like that in my floor pan under the seat. I went out and confirmed when I replied last night.

Makes sense if it's some sort of hold-down slot for something (jack, etc).

On a Windows PC, you can enlarge by holding down Ctrl and hitting +
On my iPhone, the picture opens in a new window and can be enlarged by the typical 2-finger spread/pinch thing.

Hello and thank you.

The picture is bad. My truck is in my fathers garage 40 miles away in Moose Jaw. I will pull the seat and take a real picture to get a better view.
It does look like a key slot.. It is pressed into the floor pan. I am over there in the morning and will try to snap one. Thanks for your input. Brent

Didn't Denny Graham make a plug to fit that keyhole shape several years back? I always theorized it could have been there to hold up torque tube down the assembly line prior to engine installation or something related to manufacturing like that. It was just a guess though.

My guess is it wasn't there to hold up the torque tube, as that was probably already hooked to the transmission and engine before the cab went on. But like you, that's just a guess, too.