Development Hardware
Various development hardware was made available to 3rd Party developers and by Nintendo and used by Nintendo themselves. For the GameCube, these included units with model numbers starting with "DOT" instead of the retail "DOL". For the Wii, it is "RVT" instead of the retail "RVL". It is believed the "T" in these model numbers stands for "Tool" (Ex. DOT = Dolphin Tool (Dolphin being the GameCube development codename), RVT = Revolution Tool (Revolution being the Wii development codename)).
Since Super Mario Galaxy was being developed early in tangent to the Wii's development, there is a chance physical media exists or once existed for the game either on a GameCube NR Disc (model DOT-00?) or a GameCube NPDP Cartridge (model DOT-00?) and was thus playable on a GameCube NR Reader (model DOT-001, a greenish-teal GameCube with a special NR Disc only disc drive) or a GameCube NPDP Reader (model DOT-00?, a chunky redish GameCube with a NPDP Cartridge slot); possibly the only official instance in which the game was playable in the GameCube assembly line of hardware.
Later into the game's development, and as the Wii also neared its more final, retail form, further development and testing of the game likely took place on the main Wii development units, the NDEV (the name likely standing for "NNGC Development", further likely standing for "Nintendo Next Generation Console", with references to this name in the various Wii/Revolution SDKs, and with a model number of RVT-001, in the appearance of a black box) and with possibly dozens of digital beta/debug builds of the game built, due to the NDEV's lack of an optical disc drive; as disc images would have to be emulated through the NDEV using provided software in the Wii/Revolution SDK(s).
It is also possible the 2006 E3 Demo of the game was played on either an RVT-H Reader (model RVT-005, a red faceplate Wii with an internal Hard Drive instead of an optical disc drive) or on an RVT-R Reader (model RVT-002, a green faceplate Wii with an RVT-R Disc and NR Disc backwards compatible disc drive only) burned to an optical RVT-R Disc (model RVT-004(-01)). With the last being the more probable, as it's more economic to test showcase (or especially retail builds) on disc media, as would the consumer would do with their retail Wii.