Zodiac don't sell a genuine Zodiac A-frame for the 420, only the 4.7m which used to be called the Pro 9-Man and is now renamed the Pro 500. Strange lot - aren't they!!!
A-frames are useful if you're planning to use it as a base for an antenna, navigation lights, etc.
My only concern regards a fold-down version, is that it's still as strong, and that it doesn't rattle, otherwise it will drive you round the bend. Would be worth seeing how the genuine Zodiac A-frames attach (whether they're attached to the transom, or simply bolted to the floor), and then look around to see what else is on the market.