After an evening exposed to the elements a few things happened to my majestic monoliths.
One of them toppled over and broke; the weight of a slightly crooked post was too much for the tiny wooden stake. The concrete post broke into two pieces.
Now I have two posts left that are still in one piece.
The damp air combined with the moisture of the concrete caused the baked letters to become very spongy. Much to my surprise it was very easy to pull the letters out of the concrete, leaving their impression behind. Although most of the counter spaces came out with the letters the effect was quite nice. This was the effect that I wanted from the beginning, but I decided to leave the letters dry in place in rather than try to negotiate the setting time of the concrete.