Oh, 350 prototypes were built. Many were incomplete. The finished ones tended to go home with engineers--TI didn't exactly watch what went out the door.
I owned a 99/8 (without Pascal GROMs), so for me it was definitely real! Extended BASIC II was onboard, and showed this:
> SIZE
62235 BYTES FREE
The manuals and most schematics are long known and archived at ftp.whtech.com. I have/had paper copies of many of them. Sadly, TI never archived any Home Computer work from Lubbock, TX. Those employees had been laid off or transferred just as TI got interested in preserving archives (starting in 1983. Described in Ed Millis book: TI, the Transistor and Me.
The TI Records are at DeGolyer Library, SMU but contain very few Home Computer folders. Conspicuously missing are the papers of CB Wilson, which turned up at an estate sale and were fortunately scanned.
Wilson's office in Dallas seems to have contributed a few things to the archives. That is typical: mostly executives and Fellows were canvassed for documents. Ed Millis revived obsolete floppy formats for the cause in 1983!
But it's so riddled with typos it's extremely irritating.
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There were aproxximately 100 etched PC boards. Only 250 of these were actually assembled into working units. Out these 250, only about 150 were the final preproduction versions.
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Really. 100 boards but 250 of them were assembled?
@Shift838 made a ready configurator for the 99/4A MESS. He calls it OoeyGooey.
You might also try running Classic 99 under WINE. That works on my x86 MacBook. Classic99 has the legal license to ship with ROMs. (Well PC99 as well but it's hard to get.)
I'm not sure if JS99er runs standalone --that's a very good emulation in the browser, which you can load up anytime.
It was called the Geneve 9640 from Myarc:
https://dressupgeekout.com/geneve/
http://www.mainbyte.com/ti99/geneve/geneve.html
Wikipedia has a decent article:
https://en.wikipedia.org/wiki/Geneve_9640
12 MHz un-crippled 16-bit CPU, 80 column text, 256 colour graphics, up to 2 MB of RAM.
That would be much more promising for a Unix-like OS!
They are extremely rare these days, but a cheapo emulation would be great fun -- it's able to run most software for the 99/4A.