I see you are looking at Michael's excellent chapter on Kay guitars. This is an excellent resource.
I'm not sure what model you are referring to when you say it was built between 57-66. If you go to page 173 and look in the lower right column you'll see Michael has dated the Kay 6858 between 1966-68 which is correct.
The arched headstock profile on your guitar was only used by Kay between 1966-68. Although Michael's book doesn't illustrate achtops from this era it does picture a 1966 flat top Country II acoustic on page 162 that displays this feature. It is the left photo on the upper left of the page.
If you want to see what Spud1950 and I are looking at you may want to consider subscribing to the Catalog Library and looking at the Kay catalogs yourself.
As far as metal polish on guitar parts, I only use Flitz, non abrasive and does a great job. Just buy the small tube, it will last forever.
It's hard to say what will remove the unknown substance on the back. I generally start as non aggressive as possible and move up. My most non aggressive solvent is Rubbing Alcohol, give that a try first.
As far as value, $300 seems a little high to me for this instrument given no pickguard and changed tail piece but it all depends on where you live etc. I sold an early 60's Kay archtop about 4 years ago for $175 that was also missing the pickguard and had no case. The buyer lived in France so he had to pay an additional $60 shipping charge to obtain the guitar. So, at the end of the day he spent $225. 
I agree with you you that they have lots of volume and are fun to play. If you plan to keep it, don't worry about price, just enjoy it.Statistics: Posted by VintAxe — Thu Dec 16, 2010 3:16 pm
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