I know this has been discussed before. One of the pages in the FAM (Section 1, Sheet 3.0) has what looks like a key shaped hole, but it appears to be in a cover for the seat riser, not the actual floor.
My '51 3011 has a round hole in the bottom of the depression under the seat (in the floor proper), which is for drainage.
Can you post a photo of what you're talking about?

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.