It gets so confusing.
Here is a green 416-Z
Vintage Altec 416Z 416 Z 15" Woofer Voice of Theater Vott Speaker | eBay

I think generally the green frames were 16 ohm and the black mostly 8 ohm..... Although not always, there are 416A in green and black at 16 ohms, and 416-8A in black at 8 ohms, and in both 15 and 16 inch frames. And I bet somewhere there is a black 16 ohm 416A with a 16 inch frame.........

I'm sure all the 421s, even the 15 inch frames, had flat ceramic magnets.
The white 418 15 inch frames had Alnico.