The Cranky Professor thinks that there may be more to the story than meets the eye:
My suspicions were raised by this throwaway line in the story: She had been curator of manuscripts at two universities in America, Princeton and Huntingdon [sic].Well spotted, sir!
Hmmm. Someone who is curator of manuscripts at the Huntington Library (not university, silly Englishman) is not a naive. So I googled and read this in her obituary on a page of the Early Book Society Newsletter:
Jean’s lovely little home in Oxford was filled with treasures: Jean was the largest private collector of manuscript leaves by the Spanish forger, and she owned several important pre-Raphaelite paintings.
Miss Preston may have been the twit's maiden aunt, but she's not sounding nearly so naive as to live for 45 years with two Fra Angelico panels without figuring out what they were - the woman collected forgeries! That's someone who is well beyond mildly aware of her collection. I think she reveled in looking at the two paintings for a long, long time without ever having to increase her insurance premium. I salute Miss Jean Preston!