It's been a while since I've poked at new Pay What You Want releases (PWYW), so when I saw Terror in Flusshafen at DTRPG listed under the OSR category, I figured I had to give it a peek.
Well, it may be OSR in attitude, but it seems like a mishmash of systems in reality. There's no AC, but there IS a DEF score. It uses Luck and Advantage/Disadvantage, and claims compatibility "with most OSR rule sets including Labirynth Lord, Lamentations of the Flame Princess, OSE, 5E Harcore Mode, and Five Torches Deep." It very well may be compatible, but you'll need to convert things on the fly for the system of your choice, and I'm not really sure what system the author is actually writing for. He claims all you need is OSR or 5e knowledge, but if you've never encountered 5e, you'll likely not have encountered Advantage/Disadvantage.
All in all, there are enough hooks and ideas to borrow liberally from Terror in Flusshafen to be worth the suggested one-dollar price tag, even if one were not to run the adventure as written (it is nominally written for low-level adventurers and includes pregens for the mystery system used). The NPCs are well fleshed out and likely worth the suggested price of admission.