My guess (WAG) is that it is a post-1962 GM replacement block, which could be machined after casting for use with a 54/55st or post 55st water pump.
It would be interesting to pull the head and see if there are the six extra coolant passages needed/used on a 261 block.
If my WAG is correct: CON 4 J190
CON 4 = Conveyor 4
J = October
19 = 19th
0 = 1970
This would be the third such GM replacement block that I have seen/hear-of.
Dave (Pre '68 Dave), what is your guess?
