Mental Floss had an interesting article called 12 letters that didn’t make the alphabet. A more accurate title might be 12 letters that fell out of the modern English alphabet.
I thought it would have been better if the article had included the Unicode values of the letters, so I did a little research and created the following table.
|Thorn with stroke||U+A764||U+A765|
Once you know the Unicode code point for a symbol, you can find out more about it, for example, here.
To enter a Unicode character in Emacs, you can type C-x 8 <return>, then enter the value.