I purchased an MDLS-20464K (specifically, MDLS20464K-LV-LED04G-G) 20x4 character LCD module from RA-Elco today for $5 hoping that I'd be able to figure out how to interface with it. I figured it was cheap enough that it wouldn't be a big deal if I couldn't.
(Sorry for the terrible pictures, all I had was my phone...)
Anyway, I didn't find a datasheet, just a diagram of the board that doesn't give a lot of useful information other than the connection pin info. I also managed to track down some initialization instructions that are much more interesting.
Apparently, however, I wasn't the first person to ever come across one of these. There is information here on how to interface with it (maybe, haven't tried it yet...) and code for a driver class is available here.
The initialization instructions mention that the HD44780 is "long gone" and that another equivalent chip is now commonly used. There is detailed information here, specifically well-written initialization information and very clear addressing information.
Szymon Kobalczyk posts information on how to use the display with a shift register (freeing up a lot of GPIO pins) here.