casting letter on 302 block

im tearing into my ford small block and found three **x casted into the lifter valley and have never seen that before. the motor is 86, out of a crown vic. anybody have any ideas

 

Ford has done this randomly since '70-ish I believe. I had a 4 bolt 351C with X's and Y's all over the lifter valley. Folklore says it has something to do with block strength, or nickel content in the iron.
As far as I know, it is nothing special or doesn't mean anything.

There has been discussion here in the past:
http://www.network54.com/Forum/119419/

Pete