Some people arrange for their postings to Google newsgroups not to be
shown in a conversation thread. They do this by using the Automatic
Removal Tool.
Is there any facility or way of viewing the missing items?

Hello willio,

There is no way of viewing these messages on Google Groups after they
have been removed. One possible solution would be for you to find
another Usenet archive where the post may not have been removed. If
the person posts messages with 'X-No-Archive: yes' in the header
however, most archives will not archive the post.

If you are interested in this approach, you might find one of these
Usenet searches useful: http://dmoz.org/Computers/Usenet/Search/

Another solution that may work depending on how old the posts you need
are, would be to purchase a subscription to a premium newsgroup
service that retains messages for a long period of time. One example
is Giganews (you can check the specific newsgroup you are interested
in to see how many days the posts are retained for here:
http://www.giganews.com/ (some groups go up to 180 days).

Please let me know if you require any clarification to this answer and
I will do my best to further assist you.

All the best,

tisme-ga

The answer told me the fact that most newsgroup archives are deceptive
in that they do not list the complete conversational thread because
people can request  their postings to be excluded. In this respect,
the answer is a disappointment - although realistic.