arcfour-hmac checksum salt value

[Kevin Coffman]

While implementing arcfour-hmac for Linux Kernel NFS use, I have run
into the following issue:

According to rfc4757 (sections 7.2 and 7.3), the salt value when
generating the checksum for both MIC and WRAP tokens is 15.  However,
the MIT, Heimdal, and Java implementations all seem to map the usage
values (used while creating the checksum) in WRAP tokens to a salt
value of 13 instead.

Can someone verify that either I'm confused, or the spec is wrong in
the case of the checksum salt value that Microsoft used for WRAP
tokens?

Thanks,
K.C.