Newer turn signals on HD have a ground wire. Black is ground, purple is positive.
On the older ones, the single wire is positive, and the lamp grounds via contact when mounted. You can see the ground in the socket routes to the inside of the lamp housing.
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R.I.P. Rubberdown. 1971-2012
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2005 E.G. Classic
S&S 510 cams
Baisley Oil pump spring
SE 3.5" pipes
S&S adj pushrods