I believe the 351 (20Hx18Wx20D)has been redesigned several times and the current model is the NR751BB. If your cutout dimensions are 20.5Hx18.5 W this fridge should fit. The specs are listed on the link.
http://www.wholesalemarine.com/p/NOR-NR751BB/Norcold+3.0+CF+Built-In+AC-DC+Refridgerator-Black+Front.htmlI installed this fridge a couple months ago and so far I'm very happy with it. It has a really cool blue led light inside and more room than the De0051 I replaced. You can mount it flush with the cabinet or have the door sticking out in a proud mount style. I couldn't flush mount it because of a thru hull for the sink behind the fridge

Also if this one works for you don't forget to order the AC cable since it doesn't come with one
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