These numbers belong to the same aircraft, as far as my research shows. The c/n filed with the FAA seems to be the problem. This bird was originally AT-6D 42-84503, then to SNJ-5 43942. The profile for N3931Z uses the c/n, while N21&RK uses the USN BuNo. Since N3931Z is no longer an active registration, can that one be fixed with the c/n to link the two?
Glenn