Slashes
If call numbers are the same up until the colon except for a slash, the correct order is un-slashed then slashed. In other words, numbers without slashes come before numbers with slashes. For example, the following are listed in the correct order:
C 51. 11: EN 8/995 (Letters before Numbers)
C 51. 11: 23 (Numbers after Letters)
C 51. 11/8: 995-96 ("Slashes" after "No Slashes")