Those are quite old and are a different size to the current grid, you may be able to source one to fit but it will involve less hair removing to just replace the whole grid.
(Put the switches in your store room too, they are always handy.)
There is a company who does universal grid dimmers which fits old MK grid in amongst others. I think it was a company called richmond, but it's been a while since I last got one.
Home automation grid dimmer, comes with about 3 plastic peices to fit that old mk one as well as other grids.
It is a slide push on/off dimmer rather than a rotary knob.
The MK dimmer is limited to 220VA so no good if you have a larger load than that.
Indeed it is Richmond that make a "universal" grid dimmer that fits many grids. They make a range of sizes, although the larger ones (700W+) take up two spaces in a grid.
You can buy Richmond at many wholesalers, and online.
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