Are U.S. special delivery stamps valid for postage?

The last four U.S. special delivery stamps were issued from 1954 to 1971.

Special Delivery Stamps - Guide to Value, Marks, History

Robert A. Siegel Auction Galleries, Inc. Sale - 968B Page 51

E14 - 1925 20c Flat Plate Perf 11 - Mystic Stamp Company

U.S. Special Delivery (postal service) - Wikipedia

E22 - 1969 45c Special Delivery - Mystic Stamp Company

· Very fine (VF) is a well centered stamp with ample margins. The following grades convey how well centered the stamp is and the width of the stamp's

USA Special Delivery Stamp- Used, VF S#E7 Light cancel

E22 - 1969 45c Special Delivery - Mystic Stamp Company

E18 - 1944 17c Rotary Press - Mystic Stamp Company

US Special Delivery Stamps 1885-1971

Illegal Use of Postage

E23 - 1971 60c Special Delivery - Mystic Stamp Company

USPS 2021 Happy Birthday Forever First Class Postage Stamps,Celebration, Invitations, Weddings, Birthdays, Anniversary, Holiday, Engagement, New Job, Retirement, Parties (1 Sheet, 20 Stamps) : Office Products

Do You Need a Postage Stamp to Send a Letter - PostGrid

USPS Global Forever Stamps - 20 Stamps : Office Products

Norvic Philatelics Blog: Special Delivery Guaranteed & Royal Mail Signed For Stamps - a definitive statement from Royal Mail