George:
I found a photo of Southern 318245 built in 1957, just a tad beyond the period you note. Because of copyright concerns I won’t paste it in here but, you can find it either by googling “photos of Southern Railway gondolas” or, perhaps, just by searching for the car number.
I had a similar project a few months back when I wanted to create an Atlantic Coast Line gondola. I ended up compromising on the car number and had to choose one that had a build date about two or three years beyond my target time frame.
Curt