It turns out that the information about system variables were at the end of the disassembly document. This is an excerpt from it:
$272,$273 Keyboard timer.
$274,$275 Cursor timer.
$276,$277 Spare counter — also used by WAIT (and printer in V1.0).
And the argument for the WAIT command is a multiple of 10 ms, so WAIT 100 waits for 1 second.
So the ...