[dmsfrr]

Not seeing or knowing for sure which distributor you have is a disadvantage but,
if you can remove and replace the plug without damaging it you should use it to oil the distributor shaft. The plastic plug is likely a cheap substitute for the original spring-loaded metal cap.

If you can post a couple photos of the inside & outside of the distributor or include the casting/part number that may help avoid confusion. Do you know that it's actually a '57 distributor, or were you told that it is? It may or may not make a difference.