This is the only photo known of a Renault R35 turret on a 251. It is seen here in la Rochelle rounding up German POW's. Most likely it is a French modification. In the picture a man stands behind the turret in the 251. The 251 is therefore not closed. The weight of the turret and the complexity of rotating it, makes it highly unlikely that it could revolve.
A reader observes: I see markings on the vehicle, namely a rectangle on the top right rear panel and what appears to be a partly obscured German cross on the stowage bins. I downloaded the image and put it into a photo editor and enlarged the area up to about 250 and it shows what looks like three of the four angles that make up the white outline style of cross.
Lets hope that a surviving observer is found to solve the puzzle!