"Reference Initials

Reference initials (the writer's initials and the secretary's/typist's initials) appear two lines below the last line of the signature block and flush with the left margin.

If only the secretary's/typist's initials are used, they are usually lowercase, e.g., rgn. If the writer's initials are included, they are in all capitals, followed by a slash mark or colon, and then followed by the secretary's /typist's initials in lower case, e.g., TLM/rgn or TLM:rgn.

If the writer is different from the person sending the letter, the signer's initials should appear first, the writer's initials should appear next, and the secretary's/typist's initials should appear last:

EBD/TLM/rgn
EBD:TLM:rgn"

http://www.kcitraining.com/styleguide/letelem.html