If you think about it, it makes sense that SMT parts would often have bottom markings for bottom-vision pick and place machines
But sir, that involves thinking, and we'll have none of that when I can produce magic smoke in quantity like this! But yes, now that I know the marks are there, I'll know what to look for. I learned a LOT from this first time out (I'm getting/making a vacuum pen after squirting some of these LEDs across the room with tweezers never to be seen again, I'm not trying a hot-plate reflow again until I have my sous vide controller modded to avoid the big jumps I was getting toggling the control by hand with a digital thermometer, I now know about the markings, I also know that daubing solder paste onto pads with a wire actually works but you need a lot less than I even thought...). Very very informative.
Oddly I wasn't able to get my DMM to register anything with the LEDs; I assume they just didn't have enough contact surface to register. I'll play more.