Request clarification of RFC 2440 algorithm list

[Ryan Malayter]

This is probably a Werner question, but someone else might know.

In RFC2440, one of the symmetric algorithm choices is ID=3D6, "Reserved
for DES/SK"

What does the SK mean? "Single Key" would seem to be obvious, but I
can't find this specified anywhere. I assume it's not an abbreviation
for "Fanclub des SK Rapid Wien", which came up in my Google search.

If it is indeed single-key, 56-bit DES, I have to ask: why? To let
OpenPGP implementations to comply with US export law?

Regards,
:::Ryan Malayter
:::Bank Administration Institute
:::Chicago, Illinois, USA
:::PGP Key: http://www.malayter.com/pgp-public.txt