Some of the questions raised were related to the position of the digit 5 in both the pre-64 FWT 358 and on a 375 H&H. Here are a couple of the examples of those rollmarks on pre-64s I looked at this week and that I know left the factory chambered as stamped and currently remain so.
First one is a FWT 358
The other is a transitional model DOM 1948 375 H&H rollmark
I’m posting because I find it curious that the 358 rollmark is quite aligned, while the digit 3 on the 375 is a bit smudged. Before this latter photo starts an argument about the authenticity of the 375, here’s the original box
My main point in posting these photos is to show that mechanical processes performed on an assembly line are prone to variation. I don’t know whether the rifle in the OP is authentic, but I wouldn’t make a definitive determination based on the cartridge stamp alone.