That blank pad at the distributor usually indicates that the block is a replacement item, one that a dealer would stamp to match the failed engine they were removing. Since some stares used the engine number as a vehicle ID number, that was necessary to keep the paper trail correct on the vehicle. Does the engine have the "captain's bars" casting, two vertical bars cast into the block just above the starter area? Those marks, along with a similar pair of bars cast into the block on the opposite side of the engine, and the big oil ports in the latest-model 261 are all ways to distinguish the 261 from the similar-looking 235's. The absolutely positive way to identify a 261 is to pull the cylinder head and measure the bore size. 235's have a 3 9/16" bore- - - -261's are 3 3/4". Both engines run the same crankshaft- - - -a 3 15/16" stroke.
Jerry
