This firm produced perfins from 1917 to 1929.
Here are some SEC covers produced in that time:
Used 12 April 1929 to Vienna, Austria
SEC perfin on 2¢ Máximo Gómez definitive (1925) - Scott 275, Edifil 215
SEC perfin on 5¢ airmail definitive (1927) - Scott C1, Edifil 222
This envelope, courtesy of Mark Tyx, has an SEC perfin on an envelope sent apparently for personal, perhaps philatelic, use. The sender was an active Cuban philatelist – L. de Leon – a woman, Maria Luisa de Leon. She prepared a number of Cuban flight covers and you see her envelopes in her philatelic correspondence in the 1930s. How she procured an SEC perfin stamp is unknown.
Used 17 September 1928 to San Francisco, California
SEC perfin on 5¢ airmail definitive (1927) - Scott C1, Edifil 222
Used 30 October 1928 to Boston, MA. SEC perfin on 2¢ Gómez, Scott 275, Edifil 215 (1925).
Here are some of the stamps which have SEC perfins: