Not to be contrarian but where is "Castle Lock" an actual chess term. I've looked and can't find the term anywhere.
I'm not convinced this is the proper decode.
I know that Castle meant White House during JFK's presidency.
Also Q has used the term Lock and the term Key several times before. Indicating codes or puzzles (pic related)
So when he said "Castle LOCK". I believe he meant look to white house for part of the puzzle. The flag of course an added the clue of what to look for. This pointed us to Trump [WH or Castle] event displaying flag on or around the date of that post. Leading us of course to that event the day before of the prisoner release and the Q74 giving us confirmation of the post and of Q's direct link to the POTUS.
Ultimately it seems he is training new people as well as some older how to read into coded messages and how to find proofs.
This seems important as further clue drops must be eminent. Also any deleting by Potus or by Q should be a big red flag that it's important.