This should help:
NAC 52 - Significant Changes by S/N
Major Production Changes in NAC 52 Production History.
*** We cannot economically carry out the “Unity Gain” modification on early (pre S/N 79143 ) 52s.
RC5 Remote Control Operation
From May 1992 (S/N 78817)
*** Main Board Revision 3 (latest)
From Oct 1992 (S/N 80965) links fitted in place of link boards.
Pots 7 (Volume & Balance)
From Nov 1996 - To Nov 1997 (S/N 126147 - S/N 137809)
*** Pots 8 (Volume & Balance)
From Dec 1997 - To May 2003 (S/N 138507 - S/N 195336 End of Production)
Logic Board PIC Control issue 7
Small extra functionality with Flash Remote From Dec 1998 (S/N 149934)
Owners of 52 with pots earlier than Pots 7 (S/N 126147) could benefit from an upgrade to pots 8.
We change the number of tie wraps applied to wiring looms. This is a simple and worthwhile modification.
The last price we had for the control board replacement was around £1,200 and we did not recommend it. We now do not offer this.
https://forums.naimaudio.com/topic.php?oid=1566878604356287&coid=159503632588890