tyburn ticket

from The Collaborative International Dictionary of English v.0.48

Tyburn ticket \Ty"burn tick`et\ [So called in allusion to
   Tyburn, formerly a place of execution in England.] (O. Eng.
   Law)
   A certificate given to one who prosecutes a felon to
   conviction, exempting him from certain parish and ward
   offices.
   [1913 Webster]

from Bouvier's Law Dictionary, Revised 6th Ed (1856)

TYBURN TICKET, Eng. late. A certificate given to the prosecutor of a felon 
to conviction, is so called. 
     2. By the 10 & 11. W. III., c. 23, the original proprietor or first 
assignee of such certificate is exempted from all and all manner of parish 
and ward offices within the parish or ward where the felony shall have been 
committed. Bac. Ab. Constable, C.