Hi Coin Scavenger, this definitely is an issue for proof like sets from 1960 and earlier. These sets were packaged at the mint in cardboard holders, and wrapped in a thin cellophane wrapper. The problem was that the cello deteriorated quite quickly, and thus many coins in the cardboard holders that you see today have significant toning or spotting. Many people removed the coins from the cardboard holders over the years to safeguard from toning, or to sell the higher grade coins individually.
If the package is intact and has not been tampered with, all the coins will be proof-like strikes. Be careful, because some sets have been opened and some coins replaced with business strikes. The easiest way to tell in hand is to look for bag marks. Proof-like struck coins will have significantly fewer, or none compared to their circulation counterparts since these coins were carefully produced and packaged. Other ways to tell as you mentioned is to look for square rims, deep details and mirror finish.
I am not aware of the hanging 3 variety in the proof like strikes, so check carefully for proof like characteristics. Hope this helps!
P.S. here is an example of a genuine early proof-like set in it's original packaging: http://www.ebay.ca/itm/Original-195...em4d28eefb82
If the package is intact and has not been tampered with, all the coins will be proof-like strikes. Be careful, because some sets have been opened and some coins replaced with business strikes. The easiest way to tell in hand is to look for bag marks. Proof-like struck coins will have significantly fewer, or none compared to their circulation counterparts since these coins were carefully produced and packaged. Other ways to tell as you mentioned is to look for square rims, deep details and mirror finish.
I am not aware of the hanging 3 variety in the proof like strikes, so check carefully for proof like characteristics. Hope this helps!
P.S. here is an example of a genuine early proof-like set in it's original packaging: http://www.ebay.ca/itm/Original-195...em4d28eefb82
























