Hey David,
I can't really tell from that photo, it's not that clear but maybe a field modification?
I've read somewhere that British tank crews were not happy with the internal gun mantlets of their tanks, especially the Churchill and Cromwell.
Something about the internal mantlet creating a shadow that German tank and anti-tank crews used as aiming points.
I believe the Comet was the first British tank to address this and have an external mantlet like German and American tanks.
To me, the Saumur's Churchill still looks like the museum's attempt to replace the gun.
I can't post the photo, but Wikipedia has a similar photo of a Cromwell at the Yad la-Shiryon Museum in Israeli.
The Cromwell has had external mantlet welded on and a dummy 17 pounder gun mounted.
Cheers
jjumbo