Alex
Indeed the second edition (2010) states the aircraft in fact was transferred to SAAF and also the date of its demise
I suppose research in dusty files is a never ending business and Brent's book is not final by any means. There are many other Spitfire serials not listed for instance.
Actually it was me who was interested in your 25th March list. A bit unforunate that whoever wrote their ORB considered the letters in front of the serial digits as "trash" and made no notes of them. Since the SAAF own system only consisted of digits it is understandable though....
We can also see that ORBs are not 100% reliable either or at least create questions. A bit odd, to me at least, that both flights on the 25th operate aircraft 525 coded X and 211 coded Z.
Finally comparing your interpreted serial numbers with Brent's book (2005), the following are not listed: ER525, ES211, ER116, JK233, ER941, ER983, EP666, while they are all in Spitfire International (2002). Seems pretty clear Brent had no access to the latter book when he compiled his details.
How (and when) EP659 was discovered as having been transferred to 1SAAF is beyond me, but it feels likely to have been between 2005 and 2010...
Cheers
Stig