The ELK TWIN AMP 45 output impedance is 4 ohms. (the speaker box has two 30 cm speakers. 8 ohms each in parallel so the box is 4 ohms.) 30 cm is close to 12 inches but slightly smaller. The amp uses 6CA7/EL34 power tubes, so it is a little different than a Fender which uses 6L6 power tubes. It is ok to connect another 4 ohm box, but shouldn't be needed.
You got a super deal at $ 100. The amps were never exported to the USA because of patent rights, but Canada and Europe have quite a few. They were used by a lot of professional musicians back in the late 60s and into the 70s.
Did you know there was a big fire at the company in 1969. I was there and helped clean it up. The Ladies were in tears and the guys seemed to be in a daze for a while. It happened at night , so no one was hurt. It was a sad occasion indeed to see so many burned amps and guitars. The black and white photos really tell the story.
Good luck on your amp. ELK is the only amp I used for guitar and its great to see other musicians dig them.
Regards, Joe "Fire"
sixbil@sbcglobal.net