Just for the record, and for those not familiar with these, the original ZX80 and ZX81's had only 1 Kb memory.
The "expansion" to 16 Kb (and later to a whopping 64 Kb!) was an extremely costly "add on", it was a small "vertical box" attached to the rear end connector (and the connection was - to say the least - flaky):
http://www.computinghistory.org.uk/det/34163/ZX81-16K-Byte-R...
An image of a ZX81 with the expansion box mounted is here: