Just for posterity, some notes about how TiVo does their TSN numbering.
What prompted me is the latest screenshot of the Bolt info screen. We know it's 849, but the screen showed 8F9... looking back at my TiVo Model Information
spreadsheet and wmcbrine's original first sighting of a Mini, they apparently always use 0x?F? for Evaluation hardware
... the AF2 wmcbrine saw, the 8F9 bolt, and I also sighted a 6F2 as a sample TSN on the Australian website when the real Australian models include 663.
Also, clearly they recognize they're running out of top-level numbers by giving the bolt another 84x number instead of going to 9 or even just 85x.
Finally, I haven't taken the time to decode it, but I feel 2 or 3 numbers after those first 3 identifies a sub-model. Generally drive size.
In general (with specific exceptions especially for international versions):
0 is Series 1
1,2,3,5 are Series 2
6 is Series 3
7 is Series 4
8 is Series 5 and Series 6 (and some international Series 4)
A is non-recording models (Mini, Stream, and Preview)
B is ONO (Spain)
C is Virgin Media
D is Pace
E is ComHem (Sweden)
So all they have left (without reusing like they're doing for 8) is 4 (inexplicably), 9, and F.