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 61 (page 61 of the pdf file). The letters aren't numbered but they do have a reference to a number in the Correspondence Register and a date. You will need to use a combination of the date and the reference no. to find letters that you are interested in.
The first letter is on frame 40 on Ancestry.com.
Note that we have recently added a link to the SROWA website so that you can download this register - see below.