Hi Scot, Well you got a 1964-D quarter in change. Lucky you as those are 90% silver, nice find. As to the MM issue, I believe in 64 they where stamped in by hand. So the punch had a tendancy to bounce on the second strike of the hammer. If you could post a closer photo of MM it would be helpful to anyone that is willing to give you some kind of verification. And use the full image option also.
Use the second image. Crop out all unnecessary background (including parts of the coin), resize the image and be sure when you upload the image to click on "Full Image" rather than the thumbnail. Like @alurid said, it isn't considered a doubled die because the mintmark was punched into the die by hand. If anything, it could be a re-punched mintmark (RPM). Chris
The mintmarks were punched by hand at that time, and would mean it has the potential to be an RPM. It cannot, however, be a doubled die as mentioned in the subject line. That said, I cannot see it well enough to say either way. It looks to be a possibility, but this in no way means any apparent "doubling" was automatically caused by repunching. Mintmarks can display strike doubling as well as have an odd "sheared" appearance (although I'm still not sure of the exact cause). Your best bet, even if someone else comes along claiming to see something, is simply to post better and more detailed photos. Remember, if you cannot clearly see the issue in the posted photos, neither can we.
Is the RPM for 1964 considered common? Out of the 50 or so '64's I've looked at about a third of them appear to RPM.
There is no "the RPM" for this or any date. All depends on the individual dies used and if the mintmarks were repunched or not. In some cases there may not be any known for a certain date, while in others there will be many. Check out Wexler's or CONECA to see how many are known for the 64-D 25C and compare your coins. That said, "appearing" to be an RPM and actually being one are two totally different things. I have a feeling you'll find most of your coins display something other than repunching, unfortunately.
It's still hard for me to see (likely my problem and not because of your photos), but it appears to very closely resemble the earlier-mentioned "shearing" fairly common to these. Notice that it's somewhat misshapen.
Yeah, it looks like machine doubling, also known as strike doubling, mechanical doubling and sometimes ejection doubling. On the good side, it does contain .18084 ounces of silver. Chris
Yes, it was hand-punched into the dies until about 1990, but that doesn't mean that mechanical doubling can't occur during striking. You see, during the coining operation, vibration from the machinery can cause the die to loosen in the holder. It is this back-and-forth movement at high speed that causes mechanical doubling. Chris