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.
Series 2 (Retired, sold)
TiVo HD 1TB (157 HD Hrs, PLS, "S3HD")
Roamio, Oh Roamio!
The TiVo Apocalypse begins on Sat 2013 Feb 16 00:00 GMT!