Tire strings I have left around either -and none on the light strings on the old pre-lit tree (3 years old?) we gave to my daughter this year either.
We've gone to 12 volt LED pre-lit this year - running off a 12 volt switch-mode power supply.. So like I said, I've never seen one "in the flesh" - the link above is the first I've seen, so it would appear to be far from universal. Virtually every one I've seen has a monolythic injection moulded plug - solid plastic through and through.