I have been looking at this for the last few days and have a possible decode. The underscore could be designating a missing letter. As Q has done in the past. So I started by looking for words that are scrambled within the main body of CASTLE ROCK. Being that this is in a context of part of a time line one word stands out. "CLOCK" which leaves " _ASTER". There are several words that you can make out of this using different letters for the underscore but ONE stood out to me using the letter "R".
"ARREST CLOCK"
Yes, I have seen that. But it is not spelled with a underscore. That underscore in this post is signifying a key to something. My post is only meant as a theory for discussion and it is possible for multiple meanings in a Q post.
It's called "spitballing" and it's how we roll.
Exactly.