> Not too surprising.  RFC-822 calls for only one address in the Reply-to:
header.

You caught me by surprise with this. I had checked RFC 2822, which
superseded 822 nearly six years ago.

But , pulling a copy of RFC 822 off the RFC Editor's database, I find this:

   4.4.3.  REPLY-TO / RESENT-REPLY-TO
        This field provides a general  mechanism  for  indicating  any
        mailbox(es)  to which responses are to be sent.

The same use of "mailbox(es)", optionally plural, appears in RFC 2822:

   3.6.2. Originator fields
  :
   .... When the "Reply-To:" field is present, it
   indicates the mailbox(es) to which the author of the message suggests
   that replies be sent.

In neither case is the option emphasized, but it appears to have been there,
albeit widely ignored, for a very long time: continuing my research with the
predecessor of RFC 822, I find the same description in RFC 733 (Standard for
the Format of ARPA Network Text Messages), dated 21 November 1977.

Hal Keen