Its putty, pumped from a putty gun most likely. Many boat companies bond in floors and other parts with putty pumped from a 55 gallon drum, it's catalyzed at the gun head, mixed well, then oozes out the end of the mixing tube right where you need it. You can make the putty yourself, or buy a pail, the difficult part can be getting all the putty in place and the floor down before it gets hard, longer gel times work well in this process. don't push the plywood down hard, just set it down in place. The floor can be glued down too, with a 1" gap it's a little more difficult though. If you use masking tape on each side of the stringer you can make a form (dam) to hold the adhesive in place. Again just set it down in place, don't walk on it until its completely hard. PL or 5200 will work.