Each model may have the same Tivo software, but there are slight differences. Current software for basic Roamio (TCD846500), 20.4.1-USA-1-846 for example. The ending digits tend to match the model.
Original S1 (Recycle Heaven)
Series 2 (PLS, sold)
TiVo HD (XL, PLS, "652")
Juliet Basic (4XL, PLS)
The TiVo Apocalypse begins on Sat 2013 Feb 16 00:00 GMT!