Payton-Wright had played Mary in the production that Robert Falls directed with Dennehy in Chicago not too long before the Broadway production. I was under the impression that Payton-Wright was not Redgrave's standby, that Redgrave didn't have one, but that when Redgrave had to miss several performances because of her mother's death, they put in an emergency call to Payton-Wright, who was fortunately available.
This article does describe Payton-Wright as having been Redgrave's standby:
http://discounts.theatermania.com/content/news.cfm/story/3556 So perhaps it was a case where Payton-Wright just wasn't listed as standby. I don't know.
An example from way back is when Gretchen Wyler was Lauren Bacall's standby in
Applause, but she didn't want to be listed in the playbill.
As already mentioned, apparently under those circumstances they must put an insert in the playbill if the standby is going on.