All of the Magic Pie axle threads I've seen have been M14 x 1.5mm, but M14 threads are available with several different pitches including 1.0mm, 1.25mm, 1.5mm and 2.0mm.
I suggest you check out
this post and then measure the exact pitch of your axle's thread to ensure you purchase the correct threaded nuts.
A thread pitch gauge is the easiest and most accurate way to confirm the exact pitch of a thread:
But not many people have access to one of these.
Measuring the distance over ten threads
(between 11 crests) should be accurate enough as shown here:
If yours is 15mm it will be a 1.5mm pitch, in which case these
M14 x 1.5 Nyloc nuts on eBay should fit.
But if yours only measures 12.5mm across ten threads it will be a 1.25mm pitch.
Alan