Yep, the Plus-1 expansion. Absolutely necessary if you wanted to do anything more than playing games with the Electron - with just the machine you had nowt but video and cassette interfaces, so if you needed floppies or a printer or even a joystick you were stuffed. This expansion had 2 ROM sockets (as used on the BBC Master) for instant software; I recall that languages like FORTH and PASCAL were sold this way, and I've also got a VIEW cart set. There was also a printer interface, analogue game port and I think RS232 but I can't find mine to tell you :)