I was going to point you to the thread on my Underwood "Semitic," but it looks like you've found it already. This is some great historical sleuthing! I was surprised to find Yiddish typewriters going all the way back to Yost. Thank you for the historical context! 

Good luck with your project, Nick. Hopefully you'll be able to get more information from our forum members.

It would be interesting to read your thoughts on Mark's Underwood and the other comments posted in that thread (within that thread, of course). I had a quick look at your blog and will spend more time reading it later on (it looks good), but I didn't see any source credits for all those photos that you used; I can't speak for Mark, but a reciprocal link is typically the very least you can do when presenting photographs that you didn't ask permission to use.
 

Great. I'll add the links where I got the pictures. I'm heading out to the Yiddish Book Center in the spring for a conference. I'll be sure to look at their typewriters. They don't have much information online about them, but they use their Yiddish typewriters in their advertising. I think I saw they have an industrial-sized setup used for newspaper printing. I'll be asking them for the serial numbers when I'm there.

Question--for the typewriter images I'm finding online at Ebay and elsewhere, should I include them at the typewriter database if all I have is the image and can figure out the make/model and approximate year, but don't have the serial numbers?

Not sure if you've already looked at this one, but there's currently a Hebrew/Yiddish Remington on eBay.

You're right, that is a lot of money. Looks like a factory refurbished Remington 12 (?) based on the matte/crinkle paint job. I'm also interested to see the Tytell sticker on the front -- that was a company that did a lot of top-quality customizing of machines (such as changing the typeface). They might well have converted it to the Yiddish keyboard and reversed the carriage at the same time.