All they had at the local hardware stores were 1/4" versions of this, and the table top I wanted to secure needed 6 mm cap nuts. These worked great for 11 out of the 12 bolts - the 12th one wouldn't thread due to a defect with the table top and not the cap nut. All of these looked to be perfectly crafted.

I didn't need the screw portion of the assembly but it was easier to buy the combo pack, since I wanted stainless steel.

Everything was well-packaged and delivered perfectly. I don't know why they aren't available at the hardware stores but I'm glad that Amazon has them!

Excellent quality and as described.
before purchasing I could not clearly understand what length of screw to get to clamp my item successfully. There is a range of clamping length that a screw and nut will give. This is not shown on the drawings.
So after purchasing the mix pack, the following was found.
The length of screw is given. This is the length of the screw from under the head of the screw.
The length of the nut is 18 mm from under the head of the nut. Now the thread does not run the whole length of the nut hence the need to run the nut onto the screw to see the the minimum clamp length ie the minimum distance that the screw and nut will clamp down on.

For a 40 mm screw has a minimum clamp length of 42mm. Round up to 43mm to allow error of measurement or sample differences.
Allowing for a minimum of 6 thread engagement the maximum clamp length was 49 mm.
So a 40mm screw will work from 44 mm to 49 mm. This gives a working range of 5mm.
The working range will be the same for any length of screw.
40mm screw will clamp from 44 to 49 mm
50mm screw will clamp from 54 to 59 mm
You get the idea. One could add a millimeter onto the Maximum length and still be OK but I have not done this since I prefer 6 thread engagement rather that 4.5 to 5. since the door I am using theses on are very heavy.
A quick way to convert Metric to imperial is to buy a cheap ruler with both and draw a line of the require length and measure in the preferred conversion, go to the web for a conversion table or buy a cheap vernier gauge with both metric and imperial markings.

To fix knife handle.

A bolt and cap like this is typically used to hold two pieces of wood together. In which case the distance from the inside face of the cap to the inside face of the bolt head is actually what you want to know. Be advised that this dimension is 42.9 mm on the 40 mm nominal size. If unsure, you may want to buy a kit that contains a variety of sizes.

I welded together some angle iron, and ordered the beautiful 20mm 30 pcs. kit of this cap nut-kit.

To make it clear I wanted to advise the DIY crowd"”these are super super nice in finish and quality, Thick chrome and they go together like a German-clock. To be clear: The male part is 20mm and the female part is about that same size on all the kits. For a one inch thick project these worked perfect (had to tighten all the way down).

If you are clasping together 25mm of product"”It would require the 25mm kit and so on. But I would not order a bigger size if you are using for example; 30mm or so on"”the 30mm probably clasps up to 43mms or so on...

So in review

For 25mm use 20mm. For 30mm use 30mm, for 40mm use 40mm or 30 or 35.

I hope that is not to confusing"”but just remember the "mm" measurement is the "male" part of the nut-kit and the "female" part stays the same size...