Try BlockNews and/or UsenetNews again, an hour later.

You (DFS) replied ( to me ):

I list MessageIDs (100 at a time), starting from the highest Article Number,
until I find one of the 10 MessageID "bookmarks" I've saved.

 
I used to do head and body commands (via Python nntplib), but they don't
work very well lately.  The 'article' command is pretty quick, though.
info = news.article(artID)  artID = info[0]  msgID = info[1]  msg = info[2]

Yes, instead of "head" and "body", I use the "article" command.

a minute ago, from blocknews:
    211 1795999 899518 2695516 comp.os.linux.advocacy
 
But blocknews recently screwed up their article numbering,
and there are a lot of gaps in the sequence,
so there aren't 1795999 articles available on their server.

At times, blocknews is missing some articles and such;
some seconds later, they're back.

My code actually checks that blocknews has the 10 MessageIDs
that makes up my "bookmark", in the order expected;
if it doesn't, it pays to try again later.