At some point it was considered that PFM 51, 61, 77 and MiG-21US 02 were sold to Nigeria but it seems this never happened.
Thanks again.
I consider it very likely that the four MiG-21PFM and one MiG-21 'dual' seen abandoned at Gbadolite AB (DRC) are former Bulgarian AF aircraft.
For sure, they are NOT former Serbian aircraft!