Correct. Even if there is no written offer and u buy a device / media with binaries, u are entitled to one copy of the source on demand. However many point you to sources on the web, which, although not strictly (afaik) within the licence requirements, is acceptable. If the original source is mangled, the upstream supplier has to offer the source of the mangled version.
The DSL604T source is available, but for DSL502, I did not find any. Further DSL502 has few binary files in flash for which no source is available. These files are basically CGI files, used to update flash data. I am not sure if D-link India guys ever heard of GPL.
Best Regards,
Mukund Deshmukh. Beta Computronics Pvt Ltd 10/1, IT Park, Parsodi, Nagpur-440022 Cell - 9422113746