Looks to be the oil pressure switch,S&S part number 106-0744 though it's a fair chance that it's a standard part that's widely available from motor factors and not just from S&S dealers.
Bit surprised that there's no warning light with it disconnected.
With it disconnected you will get an open circuit which would be equivalent to registering oil pressure i.e. situation normal. Bet the OP doesn't get warning light before start though which might throw up a MIL on some cars.
