Altec 816A is a combination horn-vented box enclosure with a horn in front of the driver. The horn increases the output above about 150Hz, producing the typical stepped response often found with this type of enclosures.
A datasheet for the 816 can be found at Great Plains Audio, here.
While I had access to the anechoic chamber at the university, I dragged one of my 816A enclosures there and measured it with two of the drivers that would typically be used in this box: Altec 515 and Altec 416. The curves are shown below.
The impedance is shown below:
And here are the directivity curves. It looks like the on-axis curve is a couple of dB higher all over, which is probably a measurement error. Unfortunately I can't check that, since I no longer have access to either the box or the anechoic chamber.