I had a look at the dmesg output from my 1812/513 setup. I've 13 drives, 1-4 SHR1 Volume2 and 5-8+Expansion 1-5 are SHR1 Volume1
Interesting that I would have expected to see drives sda-sdm, however what I found is drives in the 1812 chassis are sda-sdh, but the expansion unit drives are sdia-sdie.
Elsewhere in the file are references to SIL3132 devices and a 5 port multplier with a SIL vendor ID.
So perhaps the 'trick' that is part of the Synology architecture is 'chassis' disks will be sda-sd- but anything in an expansion chassis gets a new reference. Looking at the logic here it makes sense that the port multiplier in the 513 would take the next available drive reference and 'expand' it. Presumably if I added another 513 that would be sdja-sdje.