The Zeiss Telementor (63mm OG, 840mm focal length refractor) of the 1970’s is like this. If you screw the eyepiece holder directly into the telescope tube you won’t be able to achieve focus, either an extension tube or a diagonal is required. Zeiss manufactured six different extension tubes (in the range 20mm to 80mm, and usable singly or in multiple) to cover all circumstances.
‘Classic’ Newtonian reflectors of a few years ago had the opposite problem because the eyepiece tube couldn’t be too long as it would cut into the light path inside the main tube. (i.e. use of a diagonal was impossible.)
The Tomline Refractor at Orwell Park also suffers from poor prime focus image position (too far up the tube) which similarly makes the use of a diagonal impossible.
On the other hand a by-product of a ‘focal reducer’ or a ‘Barlow lens’ is a shift in the position of the focal plane image. The image is moved away from the objective with a Barlow and toward it with a focal reducer. This allows you to get the image plane nearer where you want it, but you have to accept a reduction or increase in magnification for any given eyepiece.
