Hmmmm....I have a pair of 872Bs, my first Altecs/first hit of crack in fact. I could swear that they are the newer style frame without the integral tweeter.....
Printable View
They probably are. The '75 reference guide shows 3 possible loads for 872B, two of which are in 16 inch frames.
AFAIK, the only 420 with the tweeter coaxially strapped across it is the 420Y.
http://i107.photobucket.com/albums/m...adridloads.jpg
Here's a 420-8B compliments of Wardsweb: Altec 420-8B