I'm pretty sure he doesn't have any excess prototypes left to sell. His offerings have been pretty stable for the past few years, and LED technology has matured to the point that it's no longer necessary to test a dozen different emitters to select the best one for a given flashlight.
I do remember how much fun it was when I first got into custom lights, though. It was my first foray into high-end personal gear, and my first foray into a hobbyist community. I scored a Ti PD-S with a prototype emitter in the last auction Don ever did. I still have it.
Thinking back on that time takes me to a good place in my mind, before I was constantly worrying about money, before I lived alone for a couple years while my girlfriend lived hours away (which turned out to be disastrous to my mental health), before I developed an anxiety disorder, before I spent hours every day feeling lightheaded, back when I was much more confident about my decisions. Among other things, when I bought a high-end custom flashlight, I could be absolutely certain that I had gotten the best product the market had to offer; as much fun as my RC hobby is now, it always leaves me second-guessing whether I'd have been happier with Vehicle Y instead of Vehicle X -- so to shut up the voices, I end up buying both, and then selling one for a massive loss.
It's funny, I used to think the money I spent on custom flashlights was prohibitively expensive. RC puts it into a very favorable perspective.