Yeah, they did make a side-mount bill validator to add-on to non-embedded machines. The official IGT one looked like the one in the following linked post. I haven't seen the one you posted a picture of.
http://newlifegames.com/nlg/index.php?topic=3048.msg15226#msg15226I have a couple of them for my PE+ machines. So long as you're able to obtain one with the wiring harness, they're fairly easy to install. The comm wires just harness together, and then it's just a matter of tapping into the power distribution block in the machine to give it juice.
All that said, unless you're hell-bent on getting an old S+ to take bills, if you're just sick of feeding it coins all the time it would likely be easier to upgrade to a SP chip that supports tokenization, and change the machine to something like penny denomination where you get 25 credits for every quarter put in.