Hmm. Thanks for the link Frenchy.
I've read through all this and their assessment is that the radio was fitted in the usual location, to the right of the driver. I'm not convinced.
Back to my photos again, I wish the website was still available, I could give you a link, however it isn't, I've tried; and I'm afraid I'm not in a position to upload them at the moment, so I'll try to describe what I see. In the clearest photo, you can see the tachometer quite clearly, and the back of the driver's seat, but no evidence of a commanders seat. To the right of the gun mounting extending forward is a locker containing 17 rounds of 75mm ammo. Piled on the location of the gunner's seat, is a pile of 75mm rounds in their cylindrical cases. These may be obscuring the location of the radio frame. The same series of photos shows a radio frame mounted low down behind the drivers seat on a 251/9 - this could be the location on the 251/22. The photos are all taken from vehicles in the Patton museum collection. I suspect these may be the photos in Panzer Tracts, but I don't possess the book so don't know for sure.
While I'm sure there is still plenty of room for the radio in the usual location, I don't know how you could tune it or use it if it's stuck under the gun mount. It looks as though the 17 round bin projects forward into the space for the commander. What does the kit do about the commanders station?
"Lord Raglan wishes the cavalry to advance rapidly to the front, follow the enemy, and try to prevent the enemy carrying away the guns. Troops of Horse Artillery may accompany. French cavalry is on your left. Immediate. Airey."