There is a punch that matches, and I suspect that's the reason for the outline, but it doesn't look like the typical 'punch available' type outline. For one thing the outline is barely bigger than the stamp and looking at samples, it doesn't look like that's the case. On other sets the added outline seems to match the punch better. Also, on other sets with punches available the outline is in a distinctively different color where on this one, it is blue just like the stamp images. However, looking more closely, the shade of blue is a bit different.
Wash: "A man walks down the street in that hat, people know he's not afraid of anything."
Jayne: "Da*n straight."
-- my recipes