I had to learn this on a web site about old British TV?
Ed Ellers adds:
Many American series were formatted to have a short segment at the end - whose length was equal to the difference between the NAB prime-time and non-prime-time limits - that could, if necessary, be dropped (and may not even have been related to the plot of that episode). Sadly, most stations in my experience still cut out other bits and leave in the 'appendix' segment; I've even heard of the studios themselves, when releasing old filmed series on tape in domestic syndication, editing out bits of the plot while leaving in that almost-useless final segment!
Another mystery solved: the "appendix" (a term I've never heard, but...) was meant to be removable when the show was syndicated into daytime, and replaced by more commercials.