Back in the early to mid 1980s, I vaguely recall seeing an advert in a (I think) UK based computer periodical monthly (Your Computer, or Computer and Video Games), for a memory peripheral that offered "paging" which would allow the Sinclair ZX81, or the ZX Spectrum (I forget which), to have up to 128 kB (it might have been 256 kB) of memory.
I think that the same company offered a proper keyboard as well, and the company name may have been DK Robotics1 (or something very similar to that). Or, it might have been the same company that offered an alternate [character set?] ROM for the ZX81, whose name I really can't remember2. The links provided by DroidW show that it wasn't dk'tronics after all.
I seem to remember it being a full page advertisement, showing the device, which was a large [vertical?] board full of ICs (behind the ZX81) and explaining how the paging was achieved. There may have been the ability of adding a number of boards into slots, but now my memory is getting a bit unreliable, w.r.t. that part.
I am starting to wonder that, as the board was shown not encased in a plastic shell, maybe it was a prototype. I think that I only saw the ad once. This was not an ubiquitous product, only mail order, not seen in local shops or chains. The ad may also have been in a more "serious", or business type, magazine, like PCW (Personal Computer World), and not in Your Computer or C&VG, as I first thought.
I've looked through my few remaining old magazines and can't find it, unfortunately.
I know that there is a lot of oublié, maybes and mights in this question, but does anyone recall this peripheral, and/or have a photo of the advertisement?
1 dk'tronics seems to be the keyboard producing company, although the Wikipedia page makes no mention of the expanded memory kit.
2 It turns out to be the same company, dk'tronics. Thanks to DroidW, for indirerctly providing the link to the advert for the keyboard and the graphics ROM.