NXP has published a document, Migrating from the P89V51Rx2 to P89CV51Rx2 . I'd say this might do something with the fact I repeat too often that the P89V51Rx2 are in fact manufactured by SST which has been eaten by Microchip last year.

The P89CV51Rx2 appears to be unavailable in DIP40, so this (together with AT89C51Rx51 unavailable in DIP40 for quite a couple of years) might be the beginning of end of the "biblical" packaged hobbyist-friendly '51s.

JW