In the replacement language \ has a special meaning. So if you want it literally, you need to escape it.
Your string \\\\1 is seen by the replacement language as \\1, which will result in the replace of \1 (as printed) or \\1 (as inspected).
To actually get a single backslash followed by the content of the capture, you need 3 backslashes followed by a one in the replacement language, which in a string literall have to be doubled, such that you end up with 6 of them.
When I do write replacments, I usually use ~S to avoid the duplication, then I can do ~S"\\\1".
