New Defender's Study Bible Notes
3:43 the firstborn males. The firstborn males in all the families of Israel (not including any fathers, of course) totaled 273 more than all the Levites (compare Numbers 3:39 and 43). God’s provision had been that each firstborn son would belong to the Lord, but that he would be redeemed, as it were, by a Levite–who in turn must be supported (implicitly) by that family as its representative Levite who would devote his life to God in the tabernacle (Numbers 3:41). Since no Levite were available for the extra 273 firstborn sons, the latter were to be redeemed by payment of five shekels each.