Howdy Again
Sorry for the "Rahway' comments, I misread Railway. Yes, I need new glasses.
As I said earlier, I agree your revolver is most likely a Smith and Wesson Model 11. These were shipped mostly to British Commonwealth countries, and I believe at some point South Africa was part of the British Commonwealth.
This revolver is known in the Standard Catalog of Smith and Wesson as the Model 11, also known as the 38/200 Military and Police. Military and Police is an older name for S&W 38 caliber revolvers built on the medium, or K frame.
The history of this particular model goes back to 1942 when the so called Victory Models were produced. This was a standard 38 Special Military and Police (M&P) produced for the war effort. They had a V for Victory prefix in the Serial Number. After 1944 a new hammer block was added and the Serial Numbers had a SV prefix, to indicate the new 'safer' hammer block had been installed.
Lend Lease revolvers sent to Britain and the other allies were called the .38/200 British Service Revolver, the name 38/200 signifying the 38 S&W cartridge with a heavier bullet than the standard 38 S&W cartridge. Bullet weight for the standard 38 S&W cartridge is usually around 145 grains. the 38/200 cartridge carried a heavier, 200 grain bullet. These revolvers were produced until 1945.
In 1957 S&W changed over from names such as Military and Police to a model numbering system. The old 38 M&P became the Model 10.
This photo shows a Model 10 from about 1964 at the top and a Victory Model at the bottom.
It appears to me the Model 11 was never cataloged, meaning it was not sold to the general US population, but was made exclusively for export to British Commonwealth, and former Commonwealth countries. Production of this model started in 1957 with the change over to model numbers and ceased in 1965, however some were shipped as late as 1970. A lanyard ring at the bottom of the grip was standard. Your photo of the Serial Number shows the hole for the lanyard ring, but apparently the lanyard ring itself is missing. You can see the lanyard ring at the bottom of the grip of my Victory Model.
This is a photo of the model number stamped on the frame of a Model 10, under the cylinder yoke. This is a Model 10-5, meaning that it was the fifth engineering change. I am curious, is there a model number stamped on the frame of your revolver?
Sorry, your revolver was not made before 1950. As I said earlier, the serial number range for 1961 through 1962 was C474149 through C622699. Your revolver falls into that SN range. I can only guess that C531810 is early in that range, so perhaps 1961.
No idea what the GD or PS stamped on your revolver mean. They are hand stamped and I doubt if it left the S&W factory with those stamps. Perhaps an armorer's rack stamps.
I am very intrigued with your rear sight, and would like to see a clear photo of it from the top. It does not appear to be a standard S&W sight, perhaps a custom addition.
Hope this helps.