The date code on the watch shows Feb of 71
http://www.watchsleuth.com/seikodatefinder/?mvmt=6139&case=7020&serial=125351
Feb of 71 would be a resist dial and case, the first of the non-notched case, and one of the last 6139a movements.
It reads as OK to me, minus the dial. I am using this as a guide for it
https://thespringbar.com/blogs/guides/the-seiko-6139-600x-collectors-guide
It does seem to read via that website that the dial is post 75. But that might take more research it is a suwa dial though which might help you drill it down more. It is possible that this one has a later dial on it. My guess is that the low price you paid for it, is a sign that the Seiko collectors identified the later dial as not being original to the piece.. either way you got a good deal IMHO, and you can hunt for the correct dial.
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