My Radiomobile Installation Manual only covers XK120 but the brackets that Rob pictured are the same as the ones on my 140 DHC so I am fairly sure that the installation will be the same.
The 4260 is a development of, and is very similar to the 4200. I think Smiths thought the styling on the 4200 was a bit boring and blinged it up a bit. Electrically, they are almost the same. I have checked the cases of one of my 4200s and one of my 4260s and the three 2BA mounting points are in the same place.
Unfortunately, there is no picture of the 4260 bracket but it is likely to be shaped to pick up the three mountings in the same way as the alternative control unit mounting for the 4200. It will have holes that align with the drawer slide mounting holes.
Although not quite the complete information you were seeking, there is probably enough here that can be interpreted to make up the bracket you need.
I have large box of mounting brackets that came with radios I bought. When I have a few moments I will have a look and see if there is anything that looks like it might work but I don’t recall seeing one for this model.
BTW, did your radio come with the very long nine way cable? You were lucky if it did. I have lots of short ones but have never seen one of the correct length for a 140.