Mint marks were suspended from 1965 to 1967 due to the changeover from silver to clad coins, so it's perfectly normal for all 1965 halves to be "plain", with no mint mark. The letters most likely were added by someone using a metal punch, which makes the coin a damaged item with no collector value. In any case, it's only 40% silver so its melt value is roughly 1/7 of the price of an ounce of silver. With silver in the $15 range as of this writing (mid-2016) that's roughly $2.
1965 to 1969 Kennedy half dollars were all struck in 40% silver and have a bullion value of about $2.00
There are no mintmarks on 1965 Kennedy half dollars.
See related answers.
As of 19 March 2016, 40% silver Kennedy halves (1965-70) are worth $2.34.
Uncirculated 40% silver Kennedy half's are very common. Average retail values for most are $4.00-$4.25.
Kennedy half-dollars dated 1965-1970 contain 40% silver and 60% copper, with a total weigh of 11.5 grams. Using metal prices as of 17 February 2013, the copper in one 1965 half-dollar is worth 5.7 cents. The total melt value of the coin (silver + copper) is currently $4.47.
The US mint did not use mint marks on the 1965 half dollar coin.
The 1965 Kennedy half dollar is a 40% silver coin (1965-1969) but so many were made only a very high grade uncirculated example is worth more than the silver in the coin. The silver value today is about $2.70
All Kennedy half dollars struck for general circulation from 1971 to date are made of 75% copper & 25% nickel not silver and are only face value. 1964 are 90% silver and 1965-1970 40% silver.
The 1964 Kennedy half dollar contain 11.25 grams of silver. Kennedy half dollars from 1965 through 1970 contain 4.60 grams of silver.
Kennedy halves from 1965 to 1970 were struck in 40% silver, so yes.
Yes, even though these are only 40% silver they are still worth more than their face value. The current value is about $2.82. This value will change as the price of silver changes.