Quarter Wave Transmission line impedance

I read a few reviews of various Castle 1/4 wave TL speakers on old HiFi World magazines that have appeared on the American Radio History site after the magazine closed.
They are clear that these are 1/4 wave and the impedance curves show a mild single hump, definitely not a reflex double hump.
I had thought that only half wave did this.

My own TDL T-Line 3s measure with a single hump and look like they might be 1/4 wave from the size
What Dave said. Just to expand a little -any line sealed at one end & open at the other is a QW variation. The geometry of the pipe as well as its axial length determines the fundamental frequency, but it's still a QW resonance.

An undamped QW will show the usual double-saddle impedance curve of any other vented enclosure type, but with a decaying resonance chain in line with the harmonic modes of the pipe. Those are the first to be suppressed with damping materials, followed progressively by the lower of the main impedance peaks & to some extent the upper.