These strange looking bolts are known as "elevator bolts" in the fastener industry.  They are the best fastener to use when doing floor replacement in vintage travel trailers.  They have wide flat countersunk heads, so they are idea for attaching plywood to your steel chassis frame.

These are steel 1/4-20 zinc plated bolts.  Use them with inexpensive 1/4-20 zinc plated hex nuts (not included) from your local hardware store.

Use a Forstner bit to countersink the head fully.

Sold in packages of 50.

Choose from either 2" or 1-1/2" lengths.