A bunch of small random thoughts emerge in response to this thread. In no real order:
First topic (annoyingly long):
I vaguely recall that Apple released two different serial cables in the early-Macintosh era (by which, in this case, I more or less mean "the span when at least some currently-selling Macs still had DE-9 serial ports"). One of the two cables would have been used with an original ImageWriter, and the other with newer peripherals.
(Full disclosure: I have never actually gotten my hands on an ImageWriter 1, but I did get onto a "figure out what cables Apple actually supplied around then" kick for a while last year, so I'm not completely full of smoke here.)
Anyway, reading between the lines, it seems likely that the earlier cable was of the "RS-423 to -232" type, while the newer ones would have been of the plain "RS-422" type. To explain what that actually means, first, here are three sentences of vital background data: {1} RS-232 sends individual bits by making the signal line positive or negative with respect to ground (which works great unless the "ground" at each end are not the same voltage), and is asymmetrical (i.e., connecting the cable backwards can't work). {2} RS-422, which Macs use, is similar but fixes both of those: as it sends each bit, it sends the opposite of that bit on a second wire (so there's always a clear difference); and instead of the cables being wired straight-through, all RS-422 cables are "crossover" type: each end's "transmit" signals are hooked to the other end's "receive" signals (so it doesn't matter which end is which). {3} If you run a cable to the Mac's RS-422 ports but only use the _normal_ version of the data lines (i.e., don't also use the opposite-value differential lines), you get the very similar RS-423, which is close enough to the same functionality as RS-232 to be directly plugged into it (as long as you properly deal with every other signal input in the Mac's serial port that you're not actually using).
So what does that mean? Well, if I am correct in deducing that the ImageWriter 1 probably connects by RS-232, then using a normal RS-422 cable with it will fail outright in two different ways simultaneously: the ImageWriter will, from its perspective, have an unexpected null-modem cable plugged in (which _could_ have the disastrous effect of connecting the output signals of both machines to the same wire, allowing very large currents to flow and possibly cooking the serial driver ICs in both machines); and without the unused differential signals being tied off appropriately, even if it is otherwise wired perfectly, one end or the other may simply not be able to perceive the other end's output.
Secondly, I think the "it's got arrows on the plug" dealie mostly only happened with third-party gear. As far as I recall (_not_ definitive!) pretty much any cable Apple ever made, at least during their beige-box period, had an apple on the D-shaped plug segments.