A duplexer is an attachment that enables the capability for double-sided
printing. Different makes/models will look different, of course, but my
HP4150 is larger in the back, since the duplexer is installed. Each add-on
is important to identify on install since it may change the exact driver
you use. It was a common problem that I saw years ago where a user could
not print on both sides due to a generic driver being used.