We have a scanned copy of the index for this file which you can download and then look up to work out which letters interest you. The "Names Index" on the index file which you can download below starts on page 56 (page 56 of the pdf file). The entries aren't numbered but they do have a reference to a number in the Correspondence Register. The subject/people entries are in order of the Corresponence Register Reference No. so you will need to do a bit of hunting for entries that you are interested in.
The first entry is on frame 510 on Ancestry.com.