For the record, to "undelete" in Thunderbird, one way is to:
1) disable the relevant filter
2) close Thunderbird
3) delete comp.lang.c.msf from your profile
Then all headers will be downloaded again...
Seems a bit much for one message. Google groups, despite it's many
flaws, is probably the simplest solution. It just wasn't sufficient in
this case, where I also wanted to say something about it myself.