Or you can drop the picture into Google image search — sorry,
Google Lens as it’s called now, and tell it to look for the source of the image. That got me, among other hits:
http://panzerserra.blogspot.com/2011...n-aa-tank.html
https://canadianarmsandarmour.ca/the-m4a1-grizzly
Both have more photos of the prototype turret, but on a quick browse through, I don’t see any mention of the “quick conversion” idea. Maybe it is in there and I just didn’t notice, of course