That number should tell what it is. I had a JPG version of some paper that Altec produced that crossed those numbers over, at one point. I lost it during a computer problem, Though. I think it was something that Bowtie posted.

I looked up the numbers on my 417s, and they're 8Hs just like they should be for the 1221A monitor.

I've also got a 416-8A...... new frame, but old motor cover. It came from Studio 28 in Grand Rapids, when it was shut down.