411s (and 413s) were introduced as the "Dynamic Force" series in '71. They originally had doped cloth surrounds, but in the same half-roll configuration as foam as seen here;

http://www.lansingheritage.org/image...ome/page19.jpg

They were always ferrite motors as seen here (also '71);

http://www.lansingheritage.org/image...ome/page25.jpg

At the time, they were Altec's only overhung VC, long throw drivers, hence the need for the half-roll surround. The typical pleated surround can't hack the needed Xmax, so any with pleated surrounds were improper recones.

Don't know exactly when the surround switch occurred, but by '75 they were foam;

http://www.lansingheritage.org/image...ome/page15.jpg