Further to Craig's answer:

"Pre 3.24": input file names were of the form [jobname]_a. Note, there is no "period" before the "_a". As of Version 3.24 input file names were of the form [jobname]._a. Note, there is a "period" before the "_a".

"Pre 5.00": means the input filename format as of Version 3.24, i.e. [jobname]._a, with the "period".

So you have these three possibilities;
Version 1.00 to Version 3.23: [jobname]_a
Version 3.24 to Version 4.50: [jobname]._a
Version 5.00 and later: [jobname].c 2

Do not, under any circumstances, manually rename or change the jobname suffix. CAESAR II uses the extension form to tell how to read the job file. All input files are upward compatible to any later version - you don't have to do anything (other than select the file.