South African Military No.4 Longbranch?

Hi, I have recently purchased a 1944 longbranch with a serial number which may have been re-stamped it is 963258, I was told that my No4 was sold to the South African Military but had seen very little use. When I purchased it it was covered in thick grease and oil, It also as stamped No4 mk1 III/3 FTR. Stamped on the bolt and inside the brass butt plate is 78. there is also stamped F50 in various places, I have'nt got a clue to what any of these markings mean so any information would be great.
Cheers.

F50 is a makers mark. Often seen on rebuilds. Your serial number is not the factory number.
Is it a Mark II configuration? Sounds like it is. Pictures tell the tale.

As stated above, pictures are worth a thousand words. Do you see the "broad arrow inside a U" that would be stamped on the knox that would denote South African use? Or perhaps the NZ /|\ (stamped or electropencilled) that would indicate New Zealand service?