Thanks for the pic. I had a 16K expansion pack for my ZX81, but it was third party, so a little cheaper than the official Sinclair product. Perhaps 30GBP rather than 50? I remember there being much chat at the time about the dreaded "RAM pack wobble". The ZX81 keyboard claimed to be touch sensitive, but was really thump sensitive! This could easily jiggle the bus connection from the motherboard to the RAM pack, causing the ZX to crash, possibly losing hours of work before it could be saved to tape. For me the solution was yet another third party peripheral: a real keyboard.