> Seyðabrævið
> This would seem to be Old Faroese for sheep letter.

It is Modern Faroese for sheep letter. The Old Norse
version would be 'sauðabréfit'.

> Does anybody know of any cognates in Old Norse, Modern Icelandic or
> Swedish for the first part of this compound, please?

Faroese: seyður
Icelandic: sauður
Old Icelandic: sauðr
Gothic: sauds (I think)
Norwegian: sau (I think)

You can see that a shift from /au/ [au] to /ey/ [EI] has
occurred in Faroese and is represented in the spelling.

> This is also an interesting link for the new course:
>
> http://www.tourist.fo/gb/tourism/themes/theme3/theme_3.htm

That's the spirit :-)

Kveðja,
Haukur