Some MIME-PEM Questions

I think I've learned just enough about PEM, MIME & MIME-PEM to become
significantly confused.  These question are based on the ID for MIME-PEM
dated 4/18/93 @ 14:00.

My questions have to do with what can appear after (or with) a header
with Content-type: message/pem-clear or application/pem-encrypted. 
E.g., the example on page 9 of the ID:

        Date: ...
        ...
        Content-type: multipart/pem; boundary="next-part"; privacy=mic-only

        --next-part
        Content-type: message/pem-clear

        Content-type: text/plain; charset=us-ascii

        <some text>

        --next-part
        Content-type: application/pem

        <PEM headers>

        --next-part--

What other headers can occur with the message/pem-clear or
application/pem headers that would have to be considered to process the
mail?  For example, is the following snippet legal?

        --next-part
        Content-type: message/pem-clear
        Content-transfer-encoding: base 64

        uihfduihbnfre
        ...

If so, then presumably one decodes the body of this part & recursively
processes it to see what it "really" is (i.e., it has its own headers &
body part).  Is that correct?

Are there other headers (besides Content-transfer-encoding:) that could
appear with the PEM-MIME Content-type headers & affect processing?

Any help appreciated.

JR