I did a search to find the Altec T/S specs, and found a link to it in the post called "Pi seven port size using Altec 421A 15" Drivers." Sadly, there is no listing for your model 406.Hopefully, someone else will reply with specs for this speaker. If not, you might have to break out the signal generator and the calculator and measure the driver yourself. Instructions and examples of the process are shown in the post called "T/S Measurements."
If you are careful, you can measure the values and get within about 20% accuracy tolerance. I think that companies using Klippel measurements probably get more accurate results than this, but it's close enough to build good sounding cabinets. I assume quite a bit of shift in operating parameters, so having perfect T/S values might be of limited value. Some tolerance is expected.