Based on a review of the literature I have on Cu-Ni alloys for marine applications, the alloys were developed back in the 1930's with deliberate additions of iron (up to 2%) to increase strength, and to increase resistance to flow impingement from flowing sea water.
Manganese added to the Cu-Ni alloys acts as a de-oxidant and de-sulfurizer. Also, manganese increases the working characteristics of the Cu-Ni alloy, and further increases corrosion resistance in marine applications.