A papal bull is a particular type of letters patent or charter issued by the Pope.
It is named after the lead seal (bulla) that was appended to the end in order to authenticate it.
This is a Papal Bulla of Pope Clement VI, dating from between 1342 and 1352, which has been deliberately cut in half. It shows the face of St Paul with the letters SP (of SPASPE - an abbreviation for St Paul and St Peter) and on the reverse shows half of the text of CLE//MENS//PP VI.
You can see the Digital Dig Team page for this find here: http://goo.gl/xmdeKh
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