OK, seriously. Imagine if I decided that there's a market - a small market, but a market nonetheless - for .410 flares in red & white.
I'll take the following logical progression:
- It is possible to legally purchase flares in other larger sizes...
therefore
- It is possible to legally manufacture flares

HOWEVER
The .410 shell is not a "popular" size, so there may not be a market for these flares large enough for a company to take on...

BUT

It may just be a matter of market size, not BATF rules.

Of course, putting a toilet paper tube on the end of your barrel and taping a spring-clamp to the barrel turns your Winchester 30-30 into an illegal "assault rifle" so I guess anything is possible.

Still, I'm amazed that the .410 flare would just drop off the face of the earth like this.