Ok,that brings up a couple of possibilities, McCartney has used both Pyramid Gold and Labella, Pyramids short scale flats are not true short scale as they are designed for bridges that use a tailpiece so are more like medium scale, I also found Pyramids a little too stiff for me, Labella are good for old school flatwound tone, when I used them I went with the extra light 760fx 96-39 gauge though the 760fl 104-43 gauge are very popular however I found the 104-43 too stiff, GHS short scale are also good for an old school Macca kind of tone, they are very slightly longer winding length (32.75" rather than the usual 32.00") so you may get the E wrapping around the tuner post but they are flexible enough that it shouldn't matter. I like very light tension strings so bare that in mind when reading the above