I've collected quite a lot of information on Altec drivers and T/S specs and I have nothing regarding the 406. Great Plains Audio does not mention anything in their lit., neither in their T/S listing or their far more comprehensive repair parts list:http://www.greatplainsaudio.com/PartsList.html
I'm thinking that you'll have to measure them yourself.
You could try posting here, too:
http://www.hostboard.com/cgi-bin/ultimatebb.cgi?ubb=forum&f=3729
Good luck.