Kasant’s 30p each

Writing a post the other day, I wanted to avoid the cliché credit where credit’s due and thought of fair dos instead. But this looked odd – I was tempted to put in an apostrophe or even an e – so I looked it up. The OED told me that it does indeed mean fair treatment and that the first recorded use was in 1859.

Fine. But what’s this? In the quotations it appears variously as [fair] doos, dew, do’s, dos and do, but in the heading to the entry it appears as fair do’s. Why did they select the spelling with the greengrocer's (or florist's) apostrophe?