@BillRM,
Ive rejected the slave count in north and the border states because the border states like Va and Md, became a source for slaves sold "South" before the Civil war. SInce the price for slaves doubled from 1807 till the start of the civil war, it became interesting that "(from Sesquicentenno site of Civil War)'That the number of slave children born between 1807 and 1860, approximated the number of slaves sold "south" during this time.
However A UNESCO report in 1977 by Dr Jose Luciano Franco identifieed the post 1807 infractions and piracy to smuggle and ship slaves in the "Middle passage" and these wound up in the Caribbean islands (mostly Cuba) and thence to the US South
I was unable to clip it due to its format. Ill clip it in the AM and send a relevant section about slave piracy during antebellum time
PS The north had reported about 9000 slaves , with most of that number in NJ and Delaware (which was actually considered a slave state like Md and Va before the war)