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I think 2many56s is correct. I've got it in may head that turn signals were not "required" until 1958. I'll have to see if I can dig up some supporting documentation on that though.
I'm fairly certain that turn signals were standard on the '57's. Standard equipment had the signals integrated into the parking lights with a 2 contact socket. Optional was a seperate fender-mounted turn signal with a single contact socket in the parking light. If you check an old parts book, you'll find a single and dual contact parking light for that reason. At least that's how it was in 1958 and I'm pretty sure '57 was the same.