Afaik, the 21 is just the older name for the 21A. Both are equivalent to the 2C-HG. There was a no. 20 for the very old Quick-Lite lamps from the 20s and 30s that was a bit smaller, but the 2C-HG works perfectly on them too.
The Gold Top was supposed to be a premium quality mantle but I think it was more marketing hype than anything else. I know that the old Silk-Lite mantels are excellent quality compared to current Coleman brand mantles.
-Phil