[quote name='mcnach' timestamp='1452342880' post='2949038']
The Jazz J&D is definitely round and 'Fender-compatible'... but yeah, it could easily be an overhang on the squareish ones. And the others do look similar.
[/quote]
Oh yes, the J&D 1975 Jazz looks very similar to the HB JB75 one. The cheaper J&D Jazz has got the square overhang 21 fret neck though, but the cheap HB JB20 Jazz has a rounded 20 fret one.
So I would guess the HB JB75 Jazz, J&D 1975 and standard Jazz, J&D P-basses are all made by J&D / Yeou Chern. The bog-standard HB P-Basses and Jazzes look to be from another manufacturer.
The J&D 1975 Jazzes come in some nice colours, and they're Alder rather than Ash. The HB JB75 ones do weigh a lot.