I got one of the S&S units- non-working, but good case. 4 chips on the MB and a "Vowel Power Word Module" plugged into the expansion port. 40 pin DIP TMC0271NCL NBS 7910, 28 pin DIP TMC0351NL NBU 7906, 28 pin DIP TMC0352NL DAU 7901, 28 pin SDIP TMC0281NL D 7906. Vowel Power has 28 pin DIP TMC0350NL CD2302 DAU 7914. I haven't desoldered them yet to see what's on the bottom.
That is a very early speak and spell; that very likely has the location test 1978 firmware rather than the final late 1979-1980-onward one. (in confusing ti fashion, both cpus are called TMC0271N; the TMC0351 and TMC0352 VSM roms on the location test and final firmware are also different with the same part numbers. Yay TI for confusingness. The newer parts are SDIP on mine though while the old location test parts I believe are DIP, as on yours.)
Can you check the back red plastic case of the speak and spell, the lower left corner should have a code impressed in it.
On my location test unit with buttons, it says: MTA1179 (USA) (1979 week 11)
The 'final firmware' one with buttons says PII8020 (Philippines) (1980 week 20)
So it is VERY likely the one you have has the location test firmware!
Later ones made after mid 1980 use a membrane keypad instead of buttons, and have HALF AS MANY WORDS! (and 1 instead of 2 vsm roms! yay for cost-cutting!)
If the speak and spell could be made to work, there's an easy way to tell if it is location test or final firmware:
Put batteries in it, and without the vowel power or other module in it, press 'module select', then press 'spell it'
If it is the 1978-1979 location test firmware (which I'm guessing it is), it will say 'spell it', then a complete gibberish word. Advancing words will cause more gibberish words to be spoken, often with impossible characters such as numbers in their spelling.
If it is the 1979-1980+ final firmware, it will not do anything (no sound at all, if no module is inserted) on pressing module select. Spell it and other modes will continue to use internal words.
Also, on the 1978-79 location test firmware, pressing module select with a module in place has no feedback at all other than all new words come from the module.
On 1979-1980+ final firmware, you will get the '4 tones' sound played when you press module select and a module is inserted.
Addendum: The Location test firmware was famously shown on the Today show (where I believe someone demonstrated the bug/lack of checking of the cart presence). I call it the 'location test' firmware since it was used in 10,000 or so units given to parents and students in several cities in Texas and other states for parent-student usage testing in mid 1978. The units were I believe sold holiday 1978 and onward, and technically the first big batches of units up until sometime in 1979 still have the old firmware.
P.S. The TMC035x VSM ROMs (AKA TMS6100 when sold to non-consumer division customers) are dumpable with an arduino/pic microcontroller/etc without decapping (though they use odd logic levels for input and output, and the power pins are odd too: VDD is -9v and VSS is 0v; see http://www.ti99.com/exelvision/website/telechargement/tms6100-80-data-manual.pdf
). I'm curious if the onboard roms and vowel power cart you have matches the ones that have been dumped already, or are different. In particular, Vowel Power was the first cartridge made, and has 2 or 3 words in it disabled in the dump we have (the word index tables are patched so those words are never called, and are replaced by duplicates of other words on the cart; the word LPC and spelling data still exist, though).