Very Unique First Day of Issue Envelope and Stamps

I came across this Unique Scott's U580 (Non Denominational Stamped envelope adorned with the 15¢ “A” Eagle - Scott 1735–36 -43 STAMPS of 1978
I believe it is the 1st Non Denominational Stamped envelope issued by the USPS.
It also contains a CUSTOMER NOITCE FROM THE USPS.

-When the postage rates increased in 1978 from 13¢ to 15¢, the first stamp with the denomination indicated by a LETTER was put into use. The stamps were prepared in case of a shortage of stamps in the uncertain new denomination. They were sold for 15¢ for domestic use only .The Series “A” were Issued in sheet (Scott 1735)- booklet (Scott 1736) - coil (Scott 1743) formats. The stamped envelop was issued as (Scott U127) - Scott’s does not list the number of Series A envelopes which received a First Day Cancellation. The three stamped envelopes preceding the Series A where U124-5 & 6.These were denominational 2cent - 2.1 cent and 2.7 cent. They averaged less than 100,000 First Day Cancellations with the lowest being 81,388. . My guess is this totally new and last minute introduction of a “Lettered Denomination Stamp Envelop” received a very very small number of First Day Cancellations.

It is rather unique in that it contains all 3 examples of the corresponding stamps as well as a Customer announcement from the USPS -explaining the reason for the issuance of these 1st non-denominational "Alphabet Stamps" with a .15 cent value.
Scotts does not listed number of this specific envelope that received 1st Day of Issue