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I think the relevant specification for PGP/MIME-signed messages is RFC 3156:
 https://tools.ietf.org/html/rfc3156#page-5
in particular:
 Note: The accepted OpenPGP convention is for signed data to end
 with a <CR><LF> sequence. Note that the <CR><LF> sequence
 immediately preceding a MIME boundary delimiter line is considered
 to be part of the delimiter in [3], 5.1. Thus, it is not part of
 the signed data preceding the delimiter line. An implementation
 which elects to adhere to the OpenPGP convention has to make sure
 it inserts a <CR><LF> pair on the last line of the data to be
 signed and transmitted (signed message and transmitted message
 MUST be identical).