The file names start with the station number and end with the codes R…, C… and S… that describe the data processing. So you just have to shift the first letter from Stefan's explanation behind the station number. The meaning of the first letter (and the respective version files) is as follows: 
R denotes the data output from a first standard correction process.
C denotes the output from additional corrections after validation. There are stations that have been rejected during validation or that have not yet been validated. These are therefore missing in the C version. 
The S version just contains some stations from Stefan's UA39_44 dataset that have been supplemented with data from surrounding stations to complete longer timeseries.

The letter codes following the first letter have the following meaning:

DC: digitised with no further corrections
DCA: Individual ascents; adjusted to daily mean, no radiation/lag correction
DCR: Individual ascents; not adjusted to daily mean, radiation/lag correction
DCRA: Individual ascents; adjusted to daily mean, radiation/lag correction
DCRD: Daily mean data; radiation/lag correction
DCRM: Monthly mean data; radiation/lag correction

For the monthly means, the criteria from the file "Criteria for monthly mean calculation of upper air datasets.xls" (see ftp server) have been used.