Please post any pics you guys can get.

Here is another consideration. The rears of the engines were often poorly covered in paint. It could be that the clamps held a cover for the distributor hole, or the distributor itself (with a cover over it to mask it) and that the engine paint simply missed most of the clamp and the surrounding area.

Study this pic of my original 71 340, all original paint, no touch ups ever. The dark areas on the rear of this engine is simply raw cast iron, no paint ever. The yellow colors you see are the block and head markings you'd see on a raw engine before it was painted.