- Type 21 has a glossy black dial; the dials of the Type 20 models are so far matte (brown, green, black ...)
- Type 21 has a lume triangle in the bezel; the Type 20 have (now) a black colored triangle at "12" in the bezel
- Type 21 has the "crown logo" (authentic to the historic Airain Type 21 models), whereas the Type 20 have the "normal" Airain logo
But for the rest: same case, same fly-back, column-wheel movement, sapphire (now, the first editions were with hesalite).
I'll check accuracy for you, but "difficult" for me as I change watches almost daily. But I have not observed any anomaly of significance so far.
Cheers, HSTE
Pic: Re-edition vs. historic Type 21