I'm by no means a Santana expert, but IIRC, many of them had a stone or slate top.

Here's a typical set of 879A's....

19


Here's another...

20

I do think you're correct in thinking that the components are the same. Here's an interestring factoid about the 420 I didn't know, copied from the LH board....

You have three kind of 420.
The 420-8a with white frame
the 420-8b with black frame from the 416 and the magnet in ticonal
the 420-8c with the frame of the 416-8B and the magnet in alnico.

Also, as you prolly know, 420's can be reconed as 416's by GPA.

Lastly, IMO, the Madrid's capture much of the Altec magic, although the OEM tweeter is essentially useless. I think a real interesting project for the long term would be optimizing a tweeter or midrange/tweeter for the Madrids.

I wonder if an 811 horn could be adapted to fit in that cabinet?

EDIT: I forgot to mention, one of the nice things about the 872B is that the front baffle board is removable, and by the above pix, same with the Santana.

It allows for some easy tinkering with the driver load while maintaining your original board in pristine condition if so desired