Long Manufacturing in NC built the balers for Ford, starting with the original Dearborn models and continuing into the later 530. They also designed the last Ford Squares, i.e. the 532/542 series, but these were reportedly made elsewhere (New Idea)for Ford. The 520 is a redesigned model 150 and was painted blue. Similarly, the 530 was a redesigned 250 and the 540 was a redesigned 350.
I have both a 250 and a 532, and while the needles are not identical, I know a 532/542 needle will mount to and work properly on a 250. Not sure about the reverse. I also know that the 530 and 540 used the same needle design as the 532 and 542, which leads me to believe these needles will also work on the 520. Since the 530, 540, 532 and 542 series were much more popular than the 520, parts machines should be much more readily available.
Check the archives of this Forum. I have seen several references to the same source for used baler parts within the past year.
Hope this helps!