HP Laserjet 6P little pieces missing at back of printer causing paper jams

[gilestoman]

We have a couple of HP Laserjet 6P printers. On one of them, two little translucent roller things have fallen out of the back and got lost. Now, when we print to this printer, the paper stops part-way out, although the print is effectively finished and we can just pull the printed page out.

Does anyone know what these little pieces are called and if it is possible to buy replacements anywhere?

I added a photo of the missing bits in place (circled in red) on our second printer.

Thanks in advance for any tips or advice!