I wanted to plug the 'g-key' on my Sankyo HMDH (equivalent to the current CF501) as I now find that, with age, the center-action has started to cause me problems. After some research I found that a reputable flute specilaist retailer in London recommended Yamaha plugs as suitable for Sankyo flutes. Thomann offered the product at a far reduced cost.
The pack contains 10 plugs, that are made from a harder plastic than I had expected, but seem well finished. They do, however, not fit a Sankyo flute. After trying all ten plugs I found one that would remain in place, if somewhat loose, the remaining nine just fall out of any of the five holes. This seems strange as they should surely all be the same size but obviously aren't.
I'm sure that this product will be fine used on the instrument they are designed for but beware if you are trying them on a different make of instrument. At least Thomanns' price made them a bargain!