This is the back of the FileStore. There are fewer connectors than at the back of the E01 because it is intended to connect to an E01 and not directly to the Econet.

The two 37-way D-type connectors are the FileStore bus in and out connectors. On its own, the E20 is connected via one of these to the FileStore bus connector on the E01. To add another FileStore, you would attach the remaining connector to the back of the next FileStore and so on, in a daisy-chain like the BBC 1 MHz Bus. If the FileStore is not the last device in the chain, then three packs of resistors on the host adapter board must be removed.

I got this lead from 8BS really to use the 37-way D-connector on it to make a proper lead to connect my two FileStores together. As it happens, the lead was a perfect size for when the E20 is sitting on top of the E01.