It's just an extended high temperature "casting bake" with incremental cool down simulate long term heat cycling (seasoning) of the castings. Some people claim that an X marker was used to identify parts that were designated for heat treatment at the foundry, but I've also seen in a thread on here where the X designation was used for separation of components on a line with similar parts being produced, i.e. 318 and 340 heads in late 1967 through 1970.