CCnnnCCa2CCCCCCCCa*

Another way to look at this is to realize that whitespace is ignored in a pack/unpack template and that template specifiers can be quantified. The original unpack template might have been written as
    'C C n n n C C a2 C4 C4 a*'
to slightly clarify just what is going where and that in, e.g.,
    ..., @ip_src[0..3], ... = unpack( ... C4 ... );
the array slice receives four unsigned bytes.