He probably didn't
http://unmamed.mameworld.info/non_atari.html
> NOTE ON PROTOTYPES:
> As many of you know, some of these 'most wanted' prototypes are in existance but haven't been made public.
> The owner acquired these rare prototypes through contacts at Atari that let him use parts to REPAIR his machines only.
> He's kind enough put his machines on display at the California Extreme show every summer.
> He's put a lot of time and money into assembling these machines, the ROMs are indeed safely backed up, and his wishes should be respected.
> At the very least, angry/begging e-mails to him will make sure that these ROMs will never be released to the public.
> So calm down and enjoy the Atari prototypes that ARE in MAME such as Sparkz and BeatHead!