| Doug, the E series speakers are high impedance types normally used with battery receivers such as the AK model 20. This type of set does not have an output transformer, so the speaker wiring is the same as a pair of headphones, usually with two standard RCA pin jacks on their ends. Speakers and headphones were optional accessories during this era, and just about any brand could be used with almost any radio. The type F speaker was manufactured specifically for the AK Model 55, which has an output transformer and speaker field. Therefore, the speaker requires a 4 conductor cable with a 4 pin plug. A non standard plug was used so that its socket could not be mistaken for a standard 4 pin vacuum tube. Speakers of this type vary widely among other manufacturers and their specific sets, so there were no non AK speaker substitutes.
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Steven 
Thomas Dermody