When was the NAC 52 modified/up-graded the last time ?

Dear Colleagues,

My search in this forum was inconclusive, so I kindly ask for your support:
When was the NAC 52 modified/up-graded the last time ?
Which year or even better: From which model numbers onwards was this implemented ?
So I can specify this in my search for a NAC 52 head unit.


Thanks & regards

Urs