Memopak 64K is a pack which extends the memory of the ZX81 by a further 56K, and together with the ZX81 gives a full 64K, which is neither switched nor paged, and is directly addressable.
The Memopak comes in an attractive custom-designed and engineered case which fits snugly on to the back of the ZX81 giving a firm, wobble-free connection. (A strip of Velcro is supplied just to be sure)