SFP GLC-SX-MM

[JoshuaEvan]

I got this number below the series of each GLC-SX-MM SFP's i had which is;
GLC-SX-MM / 30-1301-01

GLC-SX-MM / 30-1301-02

GLC-SX-MM / 30-1301-03

GLC-SX-MM / 30-1301-04

What the difference behind these numbers? anyone could help me? thanks