OK, I see. But still don't understand the part about different data for devices with different serial numbers. Is that just because you customize the data depending on who you give the unit to? I don't see how GPS firmware version would have any effect on that.
If you just want to try different firmware, it is all archived on this website. IIRC, with an old device like yours, you would change the name of the downloaded firmware file to GUPDATE.GCD and place it in the \Garmin folder on the GPS. When you restart the device, it should ask if you want to install older firmware if a newer version is already installed.
http://www.gawisp.com/perry/nuvi/Not sure how this would help you though. Or do you want to install firmware from a completely different device on your Nuvi? Garmin certainly doesn't want you to do that and I don't think it is easy. People used to discuss this kind of thing in the GPSPassion Nuvi forum. But those days are long gone, there is very little interest in the whole field of dedicated automotive GPS anymore, let alone advanced topics like firmware hacking.