What's interesting about this is that the "PAL" 7800 BIOS with the built in Asteroids game is actually both NTSC and PAL. ;-) Though it was only previously enabled for PAL. The two new 7800 BIOS prototype's that were found are NTSC only though.
Theres more than one?
There are two released versions of the 7800 BIOS. The standard NTSC one with the Atari logo; and the one with the built in Asteroids game that is both NTSC and PAL but usually labeled as PAL only.
There are three prototype versions. One is an early version of the standard NTSC BIOS, one is a version of the standard NTSC BIOS and includes a built in copy of Pole Position and lastly there is the one that is an early version of the BIOS with the built in Asteroids but supports NTSC only.
There are also three newer dev BIOS releases but I don't know if you care about those.