Thank you for your input.
Yes, it is a 26" wheels frame.
I plan to use a chain tensioner to use it as a singlespeed.
I did look over the internet about Sibex, did not find a manufacturer site. I did find though a bunch of pictures with broken Sibex TI forms, which gave me something to worry about now
Now I am looking for the right size press-fitted bearings, which turn out to be hard to find in that dimension.

Any comments are still welcomed. Thanks.