From the U.S. Postal Service announcement on November 17, 2020:From the USPS: These stamps pay tribute to heritage breeds, preindustrial farm animals that are enjoying renewed attention for their versatility, adaptability and unique genetic traits. This pane of 20 stamps includes photographs of 10 heritage breeds: the American Mammoth Jackstock donkey, the Narragansett turkey, the Cayuga duck, the San Clemente Island goat, the Mulefoot hog, the Cotton Patch goose, the American Cream draft horse, the Barbados Blackbelly sheep, the Milking Devon cow and the Wyandotte chicken. Zack Bryant designed the stamps with photographs by Aliza Eliazarov. Greg Breeding served as art director.

Additional information will be posted below the line, with the most recent at the top.

Updated July 5th:
Here are the Scott catalogue numbers for this issue:
5583 Mulefoot hog
a. Imperforate
5584 Wyandotte chicken
a. Imperforate
5585 Milking Devon cow
a. Imperforate
5586 Narragansett turkey
a. Imperforate
5587 American Mammoth Jackstock donkey
a. Imperforate
5588 Cotton Patch goose
a. Imperforate
5589 San Clemente Island goat
a. Imperforate
5590 American Cream draft horse
a. Imperforate
5591 Cayuga duck
a. Imperforate
5592 Barbados Blackbelly sheep
a. Imperforate
b. Block of 10, #5583-5592
c. Imperforate block of 10, #5583a-5592a

Updated April 8th:
Here are the first-day postmarks for this issue:The Digital Color Postmark for this issue measures 2.96″ x 1.45″ The B&W pictorial measures 2.75″ x 1.17″

From the Postal Bulletin:

On May 17, 2021, in Mount Vernon, VA, the United States Postal Service® will issue the Heritage Breeds stamps (Forever® priced at the First-Class Mail® rate) in 10 designs, in a pressure-sensitive adhesive (PSA) pane of 20 stamps (Item 480400). These stamps will go on sale nationwide May 17, 2021, and must not be sold or canceled before the first-day-of-issue. The Heritage Breeds commemorative pane of 20 stamps may not be split and the stamps may not be sold individually.

These stamps pay tribute to heritage breeds, pre-industrial farm animals that are enjoying renewed attention for their versatility, adaptability, and unique genetic traits. This pane of 20 stamps includes photographs of 10 heritage breeds:

• American Mammoth Jackstock donkey,
• Narragansett turkey,
• Cayuga duck,
• San Clemente Island goat,
• Mulefoot hog,
• Cotton Patch goose,
• American Cream draft horse,
• Barbados Blackbelly sheep,
• Milking Devon cow, and
• Wyandotte chicken.

Zack Bryant designed the stamps with photographs by Aliza Eliazarov. Greg Breeding served as art director.

No automatic distribution.

How to Order the First-Day-of-Issue Postmark:
Customers have 120 days to obtain the first-day-of-issue postmark by mail. They may purchase new stamps at their local Post Office™ or at The Postal Store® website at usps.com/shop. They must affix the stamps to envelopes of their choice, address the envelopes (to themselves or others), and place them in a larger envelope addressed to:

FDOI – Heritage Breeds Stamps
USPS Stamp Fulfillment Services
8300 NE Underground Drive, Suite 300
Kansas City, MO 64144-9900

After applying the first-day-of-issue postmark, the Postal Service™ will return the envelopes through the mail. There is no charge for the postmark up to a quantity of 50. There is a 5-cent charge for each additional postmark over 50. All orders must be postmarked by September 17, 2021.

Technical Specifications:

Issue: Heritage Breeds Stamps
Item Number: 480400
Denomination & Type of Issue: First-Class Mail Forever
Format: Pane of 20 (10 designs)
Series: N/A
Issue Date & City: May 17, 2021, Mount Vernon VA 22121
Art Director: Greg Breeding, Charlottesville VA
Designer: Zack Bryant, Charlottesville VA
Existing Photos:: Aliza Eliazarov, Brooklyn NY
Modeler: Sandra Lane / Michelle Finn
Manufacturing Process: Offset, Flexographic
Printer: Banknote Corporation of America
Press Type: Gallus RCS
Stamps per Pane: 20
Print Quantity: 25,000,000 stamps
Paper Type: Phosphor, Block Tag
Adhesive Type: Pressure-sensitive
Colors: Black, Cyan, Magenta, Yellow, Red PMS 7621, Black PMS 7C
Stamp Orientation: Vertical
Image Area (w x h): 1.085 x 1.42 in./ 27.559 x 36.068 mm
Overall Size (w x h): 1.225 x 1.56 in./31.115 x 39.624 mm
Full Pane Size (w x h): 7.12 x 8.375 in./180.848 x 212.725 mm
Plate Size: 80 stamps per revolution
Plate Numbers: “B” followed by six (6) single digits in bottom two corners
Marginal Markings:
Front: Header: Heritage Breeds • Plate number in bottom two corners
Back: ©2020 USPS • USPS logo • 2 barcodes (480400) • Plate position diagram (4) • Promotional text

Updated April 2nd:
Why Mount Vernon for the first-day city? It has nothing to do with George Washington. It’s about the cows.

The breed shown is endangered. According to Wikipedia, “A herd of Milking Devons are currently being preserved by the Accokeek Foundation’s Heritage Breed Livestock Conservation Program within the National Colonial Farm at Piscataway Park to increase public awareness of this threatened cattle breed.” That’s a National Park, listed as being in Maryland, but the portion that has the National Colonial Farm is run by Accokeek Foundation.

That’s still in Maryland, but right across the Potomac River from the Mount Vernon, VA, post office (two miles as the crow flies).

On the other hand, you’ll pass the Accocreek MD post office on the most direct route between the park and Mount Vernon. (Technically, it’s 5 miles away, again, if you’re a crow.)Just a guess: The Accocreek Post Office may not have the staff or experience to handle a first-day.

Updated March 17th:
These stamps will be issued May 17th with a first-day city of Mount Vernon, VA.

Updated January 7th:
A mat in a suburban Washington, DC, post office indicates a issue date of May 17th. USPS headquarters does not confirm.