Via e-mail??? That's a legal order nowadays???
Back in your day, they used a horse and rider for notification. Things have changed.
Probably somewhat equivalent to VOCO, to be followed by the proper paperwork. Timeline seems kind of short though. Orders "cut" on the 23rd for reporting on the 31st. Especially given that the "Requisition" was issued on 15. May.
I was just looking at my promotion order to Captain, issued 31 Oct 84, but effective 03 Dec 83. They say VOCO confirmed. I don't recall exactly when the promotion board met, but it was quite a bit before 31 Oct. I found out via two telephone calls, one from my active duty supervisor (I was an IMA, like Major Cook) and one from my inactive duty training detachment CO, well before the paper orders arrived.