Matthew Poole and continuators on Exodus 11:5

5 And all the first-born of the land of Egypt shall die, from the first-born of Pharaoh that sitteth upon his throne , even unto the first-born of the maid-servant that is behind the mill , and all the first-born of beasts.

Either now actually, ruling with his father, as Solomon did even whilest David lived, 1 King. 1. 34. or more probably, he that is to sit, the present time for the future, he whose right this is by the custom of Egypt, and by the law of Nations.

The poor captive slave that was in the prison, as it is, chap. 12. 29. and there did grind at the Mill. In those times and places they had diverse Mills, which were not turned about by wind or water, as ours are, but by the hands of their servants, who for that purpose stood behind the mill and so with hard labour turned it about. See Iudg. 16. 21. Isa. 47. 1, 2. Lam. 5. 13.