We may get a repeat performance of the day when Vinnie pondered aloud how in the hell can a 5 1/2" replacement speaker have mounting holes on 4 3/8" centers. The holes would be in the paper cone, wouldn't they? The original speaker was sitting right in front of him and a moment spent with a tape measure could have answered the question with time to spare but I guess the keyboard was closer than the steel tape.
BTW, Mr. N4NYY: I'll assume you've found this on BAMA -- http://bama.edebris.com/download/national/nc100/nc100,101Xmanual.pdf
If not, it will be useful.
The book does not mention the speaker impedance but given the vintage the original was probably 3.2 Ohms and a 4 Ohm replacement should be the hot ticket.
What'cha got there is a fairly sexy example of pre-war engineering using classic glass-FET's. Note that with the exception of the 6C8 dual diode all tubes are single section. In one way that's good in that you won't have any issues with half a tube gone soft while the other half plays well enough to confuse your troubleshooting. OTOH, there is more point to point wiring needed for single section tubes and twice the number of tubes. Which can lead to stray capacitance issues and make it all the more important that you stick with the factory parts layout under the hood. In the book it looks like almost everything is true N-S and E-W so this is your chance to go perfectly anal. Other cool items of note are the transformer coupled phase inverter driving the 6F6's. A 6SN7 (the 12AU7's older brother) did that job 90% of the time but early Hi-Fi lore says a transformer sounds better. (?) You'll also find the 6F6's run ungodly hot so watch your pinkies in that corner of the chassis. And don't forget to nuke all waxed paper tubular condensers on sight. They are Satan's little radio tampons.
I think the sliding coil pack is mega-cool in concept but I have a hunch that unless the contact pins are of superior quality the switch can be one sweet pain in the ass at the age of 73.
No Joke... The book is dated 1940.