One way is to use tags for that, or do you do that already?
def spawnAsteroids(self):
for i in range(10):
asteroid = loadObject("asteroid" + str(randint(1,3)), scale = AST_INIT_SCALE)
if someCondition:
asteroid.setTag("isLetter", 1)
else:
asteroid.setTag("isLetter", 0)
self.asteroids.append(asteroid)
Then when you check the collision of the bullet with the asteroid:
if (collision check code from the sample) and self.asteroid[i].getTag("isLetter") == 1:
self.setExpires(bullet, 0) #Schedule the bullet for removal
self.asteroidHit(i) #Handle the hit