All edit boxes histories are stored in your configuration storage, which is by default located in Windows Registry (unless you have switched to an INI file).
The Registry key is:
HKEY_CURRENT_USER\Software\Martin Prikryl\WinSCP 2\Configuration\History\Mask

In WinSCP GUI you can only delete whole history, using Ctrl+Del, while the particular history is dropped down.

I create a lot of one-time file masks to speed searching for files. How do I delete the unneeded ones? Replacing an old file mask with a new one using Edit doesn't work--the new mask is created but the old mask is retained. So where does the mask file reside? I can't seem to find it but I figure if its a text file I could edit it. Thanks in advance.

Using version 5.14.4.

John