Sorry, haven't been around for awhile. No, it isn't a Mustang jack, or at least not for the early years (65 through the Mustang II's). I haven't seen every variation yet I would guess, but this one is not one I've run into and I have a pretty good line on all the early jacks.
The handle is similar to those for 68 up as yelostang states. There were a couple of variations on that handle, one the hole is "flat" and one with the hole curved, when you see them together you'd understand. I've been working (slowly) on some comparison pictures.
The handles and jacks for a lot of manufacturers were the same, so this could be for a Chrysler or GM product. If you turn the flip top up on the handle, look inside for a stamp. I picked up a Chrysler handle the other day that looks almost exactly like that one, but did have a Chrysler stamp underneath.
Because it doesn't have any indentations for the pinch weld, I would suspect that this car might not be for a uni-body car. I've seen some jacks on e-bay advertised for Corvettes that look very similar to Mustang jacks, but with flat tops.
