I remember stating that originally, an indication of which network protocol to use was meant to go between the slashes.

I don't think so, since the double slashes only apply to Internet schemes anyway. RFC1738 says:

//<user>:<password>@<host>:<port>/<url-path>

      Some or all of the parts "<user>:<password>@", ":<password>",

      ":<port>", and "/<url-path>" may be excluded. The scheme specific

      data start with a double slash "//" to indicate that it complies with

      the common Internet scheme syntax.

But if you find another reference, please let me know.