acceptable comment lines in between pgp header and message block

have been under the impression that the only lines accetable in between the pgp
header and message block {or the pgp signature header and signature block} were:

[1] HASH identification
[2] Version identification
[3] Comment: lines

pgp 2.x also has a 'Charset:' line identifying the character set used
{this line is acceptable to all versions of pgp and gnupg}

are there any other line headings that are acceptable?
{i.e.  will not invalidate a signature if placed into a signed message}

tia,

vedaal