To determine which model of Husqvarna shelter you should order for your Automower 420 mower, it is essential to understand the compatibility of the different shelters available.

1. Shelter model with product code 587 23 61-01: This model is compatible with Automower 305, 310, 310 Mark II, 315, 315 Mark II, 405X, and 415X mowers. It is not specifically mentioned for Automower 420.

2. Shelter model with product code 585 01 94-01: This model is compatible with Automower 420, 440, 430X, 450X, 520, and 550 mowers.

As Automower 420 is not listed with product code 587 23 61-01, but is compatible with the shelter model with product code 585 01 94-01, you should order the latter for your Automower 420 mower.