This is the available aftermarket one-piece rear main bearing bronze thrust surface. It is designed to fit the engine block and replace the original cast babbitt rear thrust, as well as to integrate the crankshaft rear bearing seal.
The bronze thrust only fits the block at the rear, and not the rear main bearing cap.
Shown on the left is the bronze thrust fitted to a block, and overlaid on the right for comparison is a section of block having the original rear babbitt thrust and a machined iron crankshaft slinger seal.
The original rear main bearing seal shown was made out of machined chilled cast iron, as well as die cast aluminum, and later in 1931 from die cast zinc.