spqr - Yes, the margins are a bit tight on your 12c stamp.
Seriously great item, even if its just a front. Wagshal had those two matched bisect usages to Canada which remind me somewhat of this.
I recall only having one cover, with each of the 1c, 3c, and 12c. They are surprisingly hard items to come by. Mine also had a 10c stamp. If I can dig up a picture I'll post it. Someone on the board might own it now, who knows.
On Chip's stamp, I noticed the markings in the lower label, and I agree its definitely worth a closer look.
Happy to see a nice 12c stamp/cover thread.