Here is one on envelope 780 (scanned with envelope 782, which has a 150th anniversary of Louisiana Statehood stamp).
This particular butterfly is the Eastern Tiger Swallowtail, and was issued by USPS as a non-denominational stamp bearing the mouthful 'Non-machinable surcharge'. It is the fifth in the series, and is square to give an indication that it is intended for 1 oz letters that are square or non-rectangular. This stamp can also be used to mail a 2 oz machinable letter.
To see what others chose of their letter B, head over to the links at Sunday Stamps.