On the '55 and '56 full size cars, there were roughly 3/16" diameter factory punched holes for the rear wiring harness clips along transmission hump and rear floor panel. Aside from the larger body-to-frame mounting holes, those are the only factory punched holes in the floor panels.
The factory (depending upon which body plant it came from) usually poured a molten bituminous/tar sealant all along the stiffener tunnels in the floor panels and seams after all the resistance welding is done. And they weren't necessarily neat about it. The purpose of the sealant in the stifferners was to prevent floor covering materials (carpet or mat, whichever was used) from sagging down into these tunnels and ruining the appearance of the floor covering.
Auto restoration people can simulate this by purchasing a bucket of roofing cement and trowelling the goop into the stiffener tunnels and seams.
Years ago, when I redid my floors, I used a caulking gun and tubes of "Grundy" plastic cement on the seams. I think they went out of business, but nowadays, you can pick up a product called "Black-Jack" at the hardware store in both buckets and caulk tubes. Its the equivalent of "Grundy's". Both are asphalt-based roof patch material.