parts of a business letter, terminology question

They’re called the typist’s initials.

The typist’s initials are one of the last elements of the business letter. They include the initials of the letter’s writer in all caps, followed by a slash mark or colon, and then the initials of the typist in lower case. For example, if William Shakespeare typed a letter for his manager Virginia Woolf, the typist’s initials would be: VW/ws.

https://sba.thehartford.com/business-management/marketing/business-letter-enclosure-notation/

I’ve seen a fair number of business letters, and I’ve seen these letters only on carbons, never on an original. The correspondence of a public figure would have these letters; the person who wrote the letter may be an aide rather than the person who signed it.