Second generation hardware may generally be identified by the version number on the label located under the router. For the DSL-500 (modem/router only), Version D1 generally indicates second generation and for the DSL-504 (modem/router/switch), Version B1 generally indicates second generation.
Some early second generation hardware includes an RJ14 socket (not 9-pin serial port) for serial console access. Subsequent second generation models reverted to the 9-pin serial port for serial console access.
The DSL-500G and DSL-504G are incompatible (ie different hardware) models released in early 2004.
See also First generation hardware