It looks to me like he just used the "Forever stamp". Everyone bought them when the price went up. I did. As I recall, there was only a choice of two stamps at that time. I'm still using the single sheet I bought then. I don't even remember what the other one looked like.
I can't see any significance in the stamp he used. I think it's just the most common stamp used by everyone.
. Everyone bought them when the price went up.<<<
I am out of date on the stamps, I knew these existed, but still have many to use up on hand, are they still for sale?
If they are not still for sale, then he may have been here a while.