Also, travelstate.org describes is as a welcome letter, so I think there must be such a thing, unless I’m utterly confusing matters.
“After creating your case, we will send you instructions for processing, as well as a Welcome Letter that includes your Case Number and Invoice ID. You will use these numbers to log into our Consular Electronic Application Center ”
https://travel.state.gov/content/travel/en/us-visas/immigrate/nvc-timeframes.html