Actually, it's at the front of the blower, just behind the drive cover. You can see the stamping here on mine:
and up close:
The '417' refers to the Autorotor 417 (1.7L) case size. I don't know what the larger ones had (421, 422?). The '660' I assume is a serial number (?)
See if the guy can get a clear pic of it. Maybe it will at least tell you the case size.
And yeah, 850HP? Not with the stock fuel rails it ain't (2nd pic in your 2nd post...)
Ahh ok I see now. I may not need to ask for any more information from him because the more I find out in this thread the less it sounds like what I was hoping to buy from this guy. The car and power level sound nothing like what he described to me, and thats what had me so interested in it. Sounds more like low 400 hp to me