The PNR numbers’ starting digit does not depend on the PRS from which it has been booked.
It depends upon: Zone of the Train, w.r.t. starting station of the train.
Number Code Zone
1 SCR Secunderabad PRS
2, 3 NR,...
more... NCR, NWR, NER New Delhi PRS
4, 5 SR, SWR, SCR Chennai PRS
6, 7 NFR, ECR, ER, ECoR, SER, SECR Calcutta PRS
8, 9 CR, WCR, WR Mumbai PRS
e.g. If a ticket is booked in Rajdhani Express (Mumbai to New Delhi), as the starting station is Mumbai, PNR will start from 8.