Hej Holger!

 HG> However, if diaeresis is the same as the 'divide' sign

It is the 'o' character with two dots on top.  The 'o' character with the 
'divide' sign - I call it the slashed 'o' which hardcore encoding gurus call 
'LATIN SMALL LETTER O WITH STROKE' - is decimal 248 in latin1 and doesn't exist 
in CP437.

 HG> In Latin 1 it's represented by chr code D8

That is 'LATIN CAPITAL LETTER O WITH STROKE' and also doesn't exist in CP437.

 HG> In Latin 1 it's represented by chr code D8 or dec.216 which
 HG> happens to be the same as in CP 437.

No it isn't.  According to https://en.wikipedia.org/wiki/Code_page_437 D8 or 
dec.216 is a line drawing character and in latin1 it is 'LATIN CAPITAL LETTER O 
WITH STROKE' or character 'Ø' in utf8.

 HG> "IBM OS/2 Warp 4" "Keyboards and Code Pages"

I found a pdf online entitled "OS/2 Warp Server for e-business, Keyboards and 
Codepages" and do not see PC8 listed in there.  It does have 'Codepage 437' and 
'Codepage 819 - ISO 8859-1' and comparing them shows the same results I have 
stated above.

 HG> MK> '...' En Møøse hade en gång min syster ...
 HG> What is this .................^^ in Latin 1?

F8 or dec.248 (not a character in CP437) for the second and third characters in 
Møøse, and E5 or dec.229 (86 or dec.134 in CP437) for the second character in 
gång.  The encoding gurus call it "LATIN SMALL LETTER A WITH RING ABOVE" which 
I believe in Swedish is called the small letter angstrom.  Please correct me if 
I am wrong.

Livet är gott,
Maurice

... En Møøse hade en gång min syster ...
--- GNU bash, version 5.0.2(1)-release (aarch64-raspi3b+-linux-gnu)