I also got a 1978 TI Little Professor, which is the one where they added the On/Off and Level buttons. The chip is marked TMC1993. I decapped it and the die is marked 1990C-c3C. It looks a lot like the 0970, but it has a different pinout; all the LED connections are on one side and all the keypad connections on the other. There's no oscillator input, and the keypad rows and LED digits are on different outputs.
1990? Interesting =) I wonder what else they changed from TMS970. Is the OFF button done with discrete logic, or on-die?