Most typically 1.75” rear backing plates found on a-bodies, which would typically be SBP. Also found rarely on B-bodies (BBP of course). Same PN for the plat3, at least in 1968. As an aside, the BBP 1.75” drums were very hard to find. Impossible now I suppose. SBP easy to get at Rock Auto still. Redrill these for 4.5” PC.