Mile,
Just by happenstance I was surfing through the site 'ruhrmemories.ca' this week. It's been completely revamped and has had some new photographs added. Two of the new photographs show the rear deck of the Centurion tank with 105mm Type B barrels. In both the turrets are reversed and the barrels are clamped to deck.
After seeing these I think I know what you mean by the Depression Stop Rail and how (and why) the clamp, or crutch, had to be moved. The DSR are the rubber pads that outline the top edge of the engine deck, while the clamp was moved to the left rear side to accommodate the fume extractor.
Am I on the right track? (No pun intended.)
Cheers,
Dan.