Timestamp Converter
1355317350
Is equivalent to:
12/12/2012 @ 1:02pm (UTC)

So basicly you need to find a firmware which singature timestamp is bigger (or equal) than 1355317350. So variant 265, or any other variant which has newer timestamp. Variant 420 sw cert signature timestamp is older (even if version number is bigger), thats why it says downgrade disallowed.