None.
readreg [-e encoding] [register [filename]]
Does one of two things, dependent on number of arguments: with zero or one
arguments it it duplicates the paste buffer contents into the register
specified or entered at the prompt. With two arguments it reads the
contents of the named file into the register, just as
readbuf reads the screen-exchange
file into the paste buffer. You can tell screen the encoding of the
file via the -e option.
The following example will paste the system's password file into the
screen window (using register p, where a copy remains):