God, I can't recall at the moment off the top of my head. Thomas Henry has a great compilation of his articles and in that he discusses the difference.  It's considerably more rare than the SN76477 chip too.

Yeah I used to circuit bend a LOT. I'm responsible for all the lame circuit bent crap you see on ebay because I was one of the firsts to do it.  Pics? I have plenty. Generally with the talking teacher I had these mods:

1. Speed knob (slow and fast)
2. Tone loop (would grab the phoneme tone and loop it)
3. Noise loop (grab the phoneme but in a noisier way)
3. 3 switch that would glitch the speech code, gave you 6 variations, greatest thing was they were all not random, so if you needed a particular sound, and knew the switch settings and which button to press, you got it.
4. Double voice switch that gave the talking teacher two voices at the same time! One in lower pitch than the other, gave awesome chorusing effects when you would do the tone loops

I have pics but they are at home. I'll see if I can find any.

Ok, first off, the talking teacher has these locking tabs that are inside the frame of the TT. What you can do is take a flat head screwdriver, place it in the exact middle of the widest part of the TT. Insert, and pull up a little, you may see the little locking tab, if you push hard enough you'll be able to pull it apart, each side has one, even the stupid handle area so be careful. Good luck.

Speak & spells or any other item that I know off hand doesn't use the SN76477 chip except for the Synare Electric Drums

Here's an example
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... eName=WDVW

There are two chips out there you can look for. The SN76477 and the SN76477N one is a smaller dip size than the other.

TI also had another chip which was it's little brother called the SN94281.

You could use the chip inside the speak but you'd have to build a processor to control the phonemes that are stored on the chip. That's a lot of work unless you have a Z80 lying around. Saw an article where it depicted how to write the program to interface with the chip and make noises with it.