I know that it means that it's probably not a Kuwahara-made Apollo, unless it's a later model or one that doesn't follow the standard serial number format. All the ones I see have the serial numbers on the seat tubes.
Okay, with the frozen stem. I have tackled this before. Did you leave any stem (at least 2cm or more) sticking out the top of the steerer? If you did, it will make removal a lot easier. If you didn't, then you're s**t out of luck, short of getting it drilled out by a machine shop, which is expensive and hence not worth it.
Firstly, try Release All or Penetrine or some other lubricant and let it sit. Then clamp the fork at the crown carefully in a vice and use a pipe wrench to try and twist the stem out. If that doesn't work, you may have to make slits from the inside of the stem, down it's entire length, with a hacksaw blade. Be warned, this is TEDIOUS. Once you've made a few slits you should be able to a) get the lube to penetrate down between the stem and steerer, b) crush the remaining bits together to make some space to repeat a). Also, heating the steerer up with a propane torch may help - just don't go near the crown of the fork, because you may melt the brazing.
GOOD LUCK!
Don't give up. I worked on a stuck stem in Peugeot for months, a bit at a time. I tried everything above and more and eventually I got it out...