I believe the smaller mesh was the old size usable for all shrimp. Subsequent changes required the larger mesh for spot shrimp while allowing the smaller size to be used for other, smaller shrimp and in only shallower water; general history. Check the regs for where you are wanting to shrimp, species, open dates, and (I believe) shrimping depths.
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Remember to immediately record your catch or you may become the catch!

It's the person who has done nothing who is sure nothing can be done. (Ewing)