I have seen one FM J4B3A magneto.
I'm pretty sure when I took it a part the paw plate was stamped 35% and not 30. Someone else could have changed it.
Could also be that the kill terminal is on the left side. I have seen 4B3 with kill terminal on the left and on the right. The styled WC I think used the mag with the kill on the left side.
A J4B3 and a J4B3A are close enough they will interchange.