I had not seen the note added recently to the Scott catalogs. It is a good addition since the covers create a lot of confusion.
The official announcement card is shown at the bottom of page 1 of this thread:https://www.stampcommunity.org/topi...IC_ID=45505&
It appears to be sent by a company who sent a group under cover for remailing at MacArthur. Quite common in later FDCs. No doubt the advertising letter would have said "Here is a cool philatelic item AND let us tell you about our company too." Half commercial and half philatelic.