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I accepted the spelling offered by "get recommended" and it seems to link with EOL and NBN.
Where does the alternative spelling come from and why does iSpot prefer it even without links to EOL and NBN?
Well, this is quite interesting. I can see reliable-looking usage of both spellings on the web, and as you say, NBN uses Craesus and states that Croesus is an error. Croesus was used by Benson in the RES keys, but it's quite possible he got it wrong; this would explain the widespread use of an erroneous spelling, if so...
I'm not quite sure any more, but I think you're right: I'm not checking Leach (l8l7) and Curtis (l824) to see if they're right, but apparently Curtis introduced the error:
I'm going to assume NBN is right in this after all, and agree with yours. :o)
That sounds like two of us a bit confused now! Glad we're both prepared to agree with NBN, EOL and iSpot for the moment.
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