I was taught as a child that a widow was addressed as Mrs Jack Smith until after her husband's funeral, then from that day onwards as Mrs Mary Smith. She also moved her wedding ring from the ring finger of her left hand to the ring finger of her right hand. This, however, was not universally practised.
It wouldn't surprise me at all to learn that customs differ (and differed) from one part of the UK to another.
Nor do I see it is applicable today, where most women prefer to be Mrs. own Christian name, rather than Mrs husband's Christian name.
I use my husband's surname, as I said when we married that I didn't think I would feel married if I still was addressed by my maiden name and if anyone addressed me by my maiden name in conjunction with Mrs, I automatically waited for my mother or grandmother to answer, even after both ladies were dead!
I don't object to the few people who can't remember my married name addressing letters to DH and me by my maiden name. So far the post office has managed to deliver them, but I would be annoyed by anyone just putting my Christian name on the envelope, as I am constantly annoyed by people I don't know from Adam addressing me by my Christian name instead of Mrs + surname.