Here's a quote from a circa 1948 Stephens Tru-sonic catalog:

"STEPHENS MANUFACTURING COMPANY FIRSTS

FIRST COAXIAL SPEAKER UNIT

First to design, manufacture and sell Multicellular Coaxial 2-Way Speakers. The first Coaxial Unit is one of twelve TRU-SONIC 2-Way Speakers which have been in continuous round the clock operation at Lockheed Air-Terminal, Burbank, California, for seven years. These speakers have required not one penny's worth of servicing."

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Now I have always doubted the truthfulness of the above, as Stephens was a notorious copycat. This catalog was printed with the Los Angeles address, where they were located until 1947 or 1948 when they moved to Culver City. The catalog (I have only a photocopy) had been rubber stamped with the new Culver City address, so it was handed out soon after the move. Going back seven years as per Mr. Stephens' claim would take us to 1940 or 1941. Jim Lansing stated in his 1943 article "The Duplex Speaker" that work began on this unit at Altec Lansing in 1941. These folks are all gone now, so who knows?

The Stephens 15" coaxials began (I think) with a field coil model called the E-52A. By the time of this catalog, production had shifted to the permanent magnet P-52A. As with all their later permanent magnet versions, a single magnetic circuit energized both the high and low frequency voice coil gaps. One seeming advantage of the Altec approach was the use of separate magnetic circuits for the two driver sections. It is easy to spot the Stephens units as they have an eight cell (2x4) horn as opposed to Altec's six cell horn. Interestingly, the Stephens horns were always hand soldered sheet metal constructions, very similar to the hand soldered horn used in the first Altec field coil Duplex model 601.