There are three stainless steel firewall pieces that make up the inner tunnel that connects the two forward skins together. They need various pieces riveted to them as shown in the first two pictures.

The tunnel pieces are riveted to each other and the forward skins with a thin layer of high temp sealant between them. It’s the same sealant used in the tanks and it is meant to keep oil seepage and CO out of the cabin area.  The entire firewall is sealed with it at all the joints.

The sealant is applied and the pieces are riveted in the last two pictures.