Your probably also going to want a nice clear picture of where the initials would be. Missing designers initials seem to be highly collectible but I think they are caused by a grease fill usually, which is a pretty common error. Which youre coin might have slight signs but it's hard to tell from your pics.
Initials are there but worn, no mintmark on the obv because it was made in Philadelphia. Philadelphia didn't start using a mintmark on US coins (other then the War Nickels) until 1979 on SBA dollars and the 1980 on everything except cents.
The coin you have is beat-up, and possibly struck weakly, but that may be because of the wear on the coin. It is bad. Scratches everywhere and no luster. I can very clearly see the initials on the front of the coin, on Kennedy. That is very much there.
Philipelida half dollars just don't have mint marks, so not only is that extraordinarily common, but it is on purpose and not an error.
Now, as for a missing reverse initial. I see it faintly, and because of how bad the coin is beaten up, I'm going to say it was stuck with it, although maybe lightly, but is now so worn it is almost gone, which means its damage and not a mint error, adding no value. I could sandpaper the initials offs if I wanted to (don't), that would remove value, not add.
Face value, I'd say. Nothing is rare with the coin, just damaged in circulation.