Congrats on lucking into those. Here's the starting point in case you've not found it already:

http://www.altecpro.com/pdfs/vintage...%20Speaker.pdf

So it is officially an MI speaker (and your guitar leanings may be better placed with it).
As usual though, "if it sounds good to you... ". Searching the forum (but not 'hostboard' in total) , you will surely find info and opinions on the 421A.