There are actually two pieces to this assembly one that goes against the flywheel on the block side and the inspection cover underneath see pictures they attach with 1/4" stovebolts. This cover was often one of the first things removed/lost on these old trucks.

http://good-times.webshots.com/photo/2236263090102454676KlPtZc
http://good-times.webshots.com/photo/2938675320102454676lzIOOW