C7769-60390 Roll Feed Cutter Assembly New OEM Genuine HP for DesignJet 500 DesignJet 510 DesignJet 800 Plotter Replaces C7769-60163 and C7769-40076. 
The auto rollfeed paper cutter is installed on the slider rod and parks in the left end of the plotter. 
If your cutoff paper edge is ragged or being torn by the cutter it is time to replace it. 

System Error Code 31:XX refers to this part and page 8-32 in the service and repair manual explains the replacement procedure.
 
Automatic Paper Cutter Assembly Used in the following HP DesignJet Plotter Models
DesignJet 500 (24-inch) C7769BR, C7769B,
DesignJet 500 (42-inch) C7770BR, C7770B
DesignJet 500PS Series C7769CR, C7769C, C7770CR, C7770C,
Designjet 510 24-IN PRINTER CH336A, CH336AR,
DESIGNJET 510 42-IN CH337A, CH337AR,
DESIGNJET 510PS 24-IN CJ996A, CJ996AR,
DESIGNJET 510PS 42-IN CJ997A, CJ997AR
DesignJet 800 (24-inch) C7779BR, C7779B,
DesignJet 800 (42-inch) C7780BR, C7780B, C7780CR, C7780C,
DesignJet 800PS (24-inch) C7779CR, C7779C ,
DesignJet 815 MFP Q1279AR, Q1279A,
DesignJet Copier CC800PS Q1262A,

 C776960163, 500 cutter, 800 cutter, hp designerjet 500 cutter,

This new OEM HP part requires no exchange of the old part.