I've never before seen this mark, but I strongly suspect
it is an earlier mark of the same William DeHart listed here at:http://www.925-1000.com/americanMod_1.html
The Albuquerque, New Mexico directory for 1953 lists - "Silversmith Studio" of Wm. R DeHart jr..
William Dehart also has brief mention as being on the faculty at the Memphis Academy of Art and the New York Society of Craftsman newsletter for April 1953 contained the following notice;
Bill DeHart is to be in Charge of a nine weeks' special seminar in silver-smithing at the Cleveland Institute of Art
Notice for a 1959 Brooklyn Museum show of the American Colonial silversmith Elias Pelletreau included this text;
The purpose of this exhibition is to show the work of Elias Pelletreau in relation to major trends in 18th-century design. As a preface to the show, silversmithing technique is explained with the aid of a group of bowls by a contemporary silversmith, William De Hart, and a drawing of a silversmith’s shop done for Diderot’s Encyclopedia In 1763.
Sometime during the early 1950s, DeHart became director of design for Towle Silversmiths and is credited with responsibility for bringing about their finest modern design lines and also for bringing the modernist jeweler Earl Pardon on board as a designer for Towle.
Although all of his so far known work is in the modernist/contemporary vein, it is conceivable that as a teacher, Dehart would have had call to recreate pieces of Colonial form such as your porringer.