I couldn't find the thread where we discussed having letters in dialogue so I had to create a new one. I think Crumbly's solution was something like "A-B-C-D".
It just so happens it came up in something I'm writing. Right or wrong, this is my dialogue:
"It's Lowery Merkins. The id should be first initial, last name. L, M, E, R, K, I, N, S."
Notice how "id" is written. And when the character spelled the letters. Right or wrong, this is what seems to make the most sense for me.
And I also wrote in dialogue:
"At the FBI he worked on money laundering cases."
I wrote "FBI" as the three letters.