Magic ride, it's the nature of forums that the delivery of the witty comment gets lost. Most will have some kind of smiley face to try indicate that they might just be giving you a ribbing.
Unless you have deep pockets and buy brand new matching brand gear, picking a mast that might work can be hit and miss. Charts like the unifiber ones help make better guesses. As you have discovered, 400 + 30 doesn't equal 430 because shorter masts tend to be softer, made worse by it being a hard top. If I were you I would return the mast on account of it being hard top when the shop said Cc. You'll find closer match on most sails with CC.
As as for the serial (not cereal

) number, some brands might put it inside the base I suppose but unless you have somewhere to look it up I'm not sure how it helps?
Sometimes you can get away with the wrong size mast, some sail brands are more forgiving than others.
P.S. Pay a visit to the kite forum, that angry mob turn on each other at the drop of a hat. This is one of the more civil forums you'll find on the interwebs.