Most likely you need HJSplit > http://www.freebyte.com/hjsplit/
7z uses a similar naming, but it should look like x.7z.001, x.7z.002, etc. and you can simply extract those to join.
RAR files currently use x.part1.rar, x.part2.rar, etc. Old split RAR files used x.r00, x.r01, etc.
I...