Knoll International Saarinen Executive Chair glides  

Hi everyone! New member here.

I was very lucky to be able to buy three (very) vintage Knoll International Eero Saarinen Executive Chairs. These three were made here, in Belgium where I live, by De Coene. They had the license to produce Knoll furniture between 1954 and 1977, when they stopped producing fine furniture. 

The fabric (Hallingdal 65) is very worn and the foam has become rock hard, so I'm getting them professionally reupholstered.

Only problem is; two glides are missing from one of the chairs, and I'm afraid they're a bit peculiar. They're different from the replacement ones I already saw at https://www.design-restoration-spares.com/product/knoll-int-eero-saarinen-executive-arm-chair-glides/  I'm afraid they were only used on De Coene-production examples of these chairs. I also couldn't manage to get them off of the legs. When I looked into the legs where ones used to be, I could just see a hole (threaded?) at about 5mm deep, measuring from the edge of the tube.

If anyone could please point me in some direction for these replacements, that would be awesome.