Apparently this/these unit(s) have a trapdoor similar to the R200/R300 printers albeit in a slightly different position.
Following the instructions for the R300 should therefore work.
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Printers: (Canon) MP500/830, MX700, iP4000/4200/4300/4500/5200, iX4000(A3) (Epson) C84/86, D88, CX6600, R285/800/1900 (HP) K550, K850, K5400, L7680