These printers use the C3903A cartridge:

HP LaserJet 5 MP

HP LaserJet 5 P

HP LaserJet 6 MP

HP LaserJet 6 P

HP LaserJet 6 PSE

HP LaserJet 6 PSI

HP LaserJet 6 PXI

Canon LBP VX

Canon Series 5 MP

Canon Series 5 P