I have the DS1815+ back!
It seems to be a completely new unit, not a pot of dust or scratch on it. Completely sealed too, but I guess that doesn't say much as Synology could seal them up. However, the level of cleanliness does suggest no prior use at all.
The MAC addresses are different to mine, as is the serial number. Mine started with 165, this one starts with 157.
There is no visible resistor on the board without taking it completely apart, which I did not, but there are differences on the board that I could see.
Difference one: https://snag.gy/rP4Z5V.jpg
This shows my original on the left, and the replacement on the right.
Bottom right side of the board, which is located on the left side of the box (as the drives look at you and the ports in the back are hidden). These connectors are without any soldering in my original model, while the replacement has then all soldered.
Difference two: https://snag.gy/OvE15e.jpg
Inside the chassis, to the left of the first hard disk bay, one capacitor is different. Red one is my new one, blue was in the old one. The lettering reads: red one - FP, 52Ch, 101, 16, blue one - FP5K, 48Ch, 101, 16
The unit is up and running, migration was surprisingly fast.
However, the unit came with a different IP scheme to the one used in my house. All four ports had a 169.254.xxx.xxx scheme with all of them on the /16 subnet. That I found out by downloading the Synology Assistant.
I use a 192.168.1.0/24 so the process to set it up was not a clean one.
Plug a temporary disk into the Synology, power it on, switch IP in one computer to get a 169.254.0.0/16 address different to the four ports of the chassis, login to the box, get asked to install and realize you do not have access to the internet bcause of course not, your computer is no longer part of your network so it doesn't see the router. Switch back to the old IP scheme on your computer, download the DSM and then switch back to the 169.254.0.0/16 to talk to the box again. Then it installs it all.
Great, switch the box to the proper IP scheme of the house and switch the PC back too. Close up everything and install the disks. Power it back on and... lose it. It's nowhere to be found. Run the Synology Assistant again, and see that it has again gone to 169.254.0.0/16 (facepalm).
Switch your IP again, connect to it and get asked to install (which was frightening but part of the instructions, but why did I then have to use the temp disk and install in the first place?) and then choose "migrate" (OK, would have preferred to see the Migrate before the Install, but OK...). Now, because it migrated successfully, you do not talk to it again, because it migrated the settings too, so the unit switched to the 192.168.1.0/24 again!!!!...
Switch back to your house IP scheme and now you see it. And it works...
I wonder if the Synology Assistant could not have assisted me in changing the IP ports of the DS1815+ to the house scheme so I would not have to do much of that. At no point in the Synology KB article was any IP switching advised or suggested or guided. If I did not know networking principles, what would have happened?