This firm produced perfins from 1917 to 1939.
A perfin cover has been found and reported by its owner. Responding to my earlier statement that no covers were known to exist, Sofia Almdovar sent this to me. I am delighted to receive this proof. Here is that ENC cover produced from 1939:
Window envelope used 19 June 1939
GE perfin on 10¢ International Airmail (1931) - Scott C5, Edifil 256
Here are some of the stamps which have ENC perfins: