Startlight 35 Anode?

http://www.westmarine.com/buy/guest--dynaplate-sintered-bronze-ground-plates--P003_090_010_005


196030.jpg
 

but I cannot tell the original size or supplier due to erosion.

Then its an anode. Most likely zinc, but aluminium is a possibility.

Supplier is not important so long as the replacement comes from a reputable one and is of the correct specification material.

Size you can surely judge.

But have you not checked to see what it is bonded to. That will tell you its purpose
 
Probably find a replacement here:

http://www.asap-supplies.com/search/disc+anode

As already said it is a sacrificial anode and if you look inside the hull you should find that it is connected internally to the boats -ve system, most probably via the engine.

It should be connected to the thing it is protecting, probably the stern gear. The connection may well be through the engine/gearbox, but it is not intended to do anything to the electrical system of the boat.
 
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