Probably batches of lenses were all sent to the same region, although it is worth noting that total production is only around 1539 units, so it is not surprising that most lenses owned by NG members have serial numbers that are relatively close
Dane, a very nice lens you have there, and you show it is a very capable performer. You managed to get some good closeup shots even though the lens does not focus particularly close. Your lens is from the first sub-type with the tripod locking knob on the right. Starting around no. 181999 lenses start to appear with the locking knob on the left.