They used "brushable seam sealer". You can find it at any good automotive paint store. Use a 1/2" to 3/4" brush to put it on. You may have to cut the brush bristles back some to make it stiff enough to flow it on properly. If you look around the firewall and cowl area, you will find evidence that the factory did it that way.
Howard
I hope this is a related question, and not a hijacking... Does seam sealer do a better job of protecting joints than products like POR-15? I'm not doing a concours restoration. I just want good rust protection.