Although it isn't on the downloads page, I glanced at the paths and file names for clues. Try these URLs:

http://server.conitec.net/gsa5/updext52.exe
http://server.conitec.net/gsa5/updext24.exe
http://server.conitec.net/gsa5e/updext24.exe

You will need your A5 username and password though. They might not be available at all, but they could still be on the server.

Why don't you want 5.51, the latest version for A5?