I made an error when writing up the user attribute packets. In
section 5.2.4 ("Computing Signatures"), there is a sentence:
A V4 certification hashes the constant 0xb4 for user ID
certifications or the constant 0xd1 for User Attribute
certifications (which are old-style packet headers with the
length-of-length set to zero), followed by a four-octet number
giving the length of the user ID or User Attribute data, and then
the User ID or User Attribute data.
0xd1 is of course not an old-style packet header.
David