The '70-'71 Darts use a BA9s (SC mini bayonet) base bulbs, originally type 1895, for the side markers. The socket + wire pigtail are integral to the housing.
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The rear side markers have separable sockets, but not the fronts—this was done because the back of the front marker light is in a splash zone, while the back of the rear marker light is kept dry inside the trunk. The permanently-integral sockets up front meant no water would get in.
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If your front housing has an empty hole in it, either it's a rear housing or a hacked front one. You could try using the rear-type socket/boot 2983 545, tho it
looks like it's for a bigger-than-5/8" hole.
This is a BA9s socket designed to snap into a 5/8" hole.
While you're at it, move the (front only) side marker lights' ground wires as described
here to add side turn signal repeater function to them.