It was introduced with their .049 Sky Fury, I think.
A review in the 1954 Model Airplane News:
"An interesting glow plug is supplied. It has a very large Volume recess and an element in the form of a "W" pointing downward. During the test, conventional plugs were substituted and it was found that few gave such good results at any speed and none at very low speed, where a point could be found at which no other would sustain running unless the battery was connected."
"Test: Allyn Sky Fury .049
Plug: 1/4 x 32, Short reach as supplied, 1-1/2 volts to start; Fuel: O & R AA; Running Time Prior to Test: 2 hours; Bore: .390 in.; Stroke: .400 in.; Weight: 1-3/4 oz.
Power Prop RPM
6 x 5 10,000
6 x 4 11,000
6 x 3 12,900
5 1/4 x 5 12,200
5 1/4 x 4 13,200
5 1/4 x 3 14,300
Top Flite RPM
6 x 5 9,500
6 x 4 10,700
6 x 3 12,000
Sky Fury prop supplied 12,400
Ref.:
http://www.airplanesandrockets.com/motors/allyn-049-sky-fury-engine-review-may-1954-man.htmCox also had a glowhead for a while with a W-shaped element, as I recall.
Dave