P = first initial of the depot facility.

641087 can be a CJ2A, M38, CJ3A, industrial or early L134 wagon/truck new or replacement engine. The only way to confirm the original model chassis it was installed in is by it's serial number on the flat above the water pump. Often when these engine are overhauled the upper deck of the block will require machining to get it flat again. This machining will often remove those serial numbers. There are several magnetic and chemical methods to help make the old numbers more visible.