Is there a chance that someone accidentally renamed the document format? I've found sometimes that changing the format suffix can make the file workable (or at least when it comes to .tif, .jpg, .pdf, etc). Try duplicating the file, and renaming the suffix to a common format, and it may magically work. I've never heard of a single character format suffix.

First thing, you could try with the free utility Palantir
http://www.ios-software.com/?page=projet&quoi=16&lg=AN
in order to see what kind of file it is.

That said, if they have generated it with Font Converter, the original file(s) could be of a PostScript font, so you should have two different files.

Thanks for the help so far!
Renaming the suffix (.ps, .ttf, .otf, …) did not have any effect on being able to open the file at all, could still be simply corrupt. The single suffix .c appears strange to me, too.
The Palantir tells me:
This file may be : ASCII c programm text, with CRLF line terminators.
Any more suggestions?

.c is indeed the suffix for "c program text".
You can try to open the file with a text editor in order to see if the headings tell you something about it.
If you can, you should ask for the original file(s) and then triyng to convert it yourself.

we are getting closer, textedit shows in the headings:
C-file generated by Font converter for emWin version 3.00
This says one will definetly have to open these files in Font converter in order to do any works on them unless you get the basic documents, right?

Alright, here is how it goes: You definetly need emWin: FontConverter which costs ridiciously $2,750.00. Finally i installed a Windows on my iMac with VirtualBox. In that environment i edited the Unicodes i needed in FontConverter and copy/pasted the previously generated characters in a text editor (if you are not very careful with the correct coding you easily mess the file up). The whole thing is about getting a .c file that can be read out properly on displays.

{ /* code 0041 */
_XXX____,
X___X___,
X___X___,
XXXXX___,
X___X___,
X___X___,
________,
________};