There wasn't a SPS sub-type of the MiG-21. The SPS abbreviation is for the Boundary Layer Blowing system ( Sduv Pogranichnovo Sloya) widely known as the blowing falps. The SPS system was used for the PFM variant as the standard one. Therefore you may find also the name MiG-21PFM-SPS in a couple of references. Earlier the system was introduced to the PF version. And the sub-variant was marked as the PFS ( not SPS ) where the "S" letter just indicated the SPS system. The MiG-21SPS is the East German name used only for their PFMs and just by themself only. What is more the German PFM with the GP-9 gun pod was marked with the SPS-K name. It happened because the NVA ( Nationale Volksarmee ) the National People's Army of the German Democratic Republic decided to use the marking to differentiate the MiG-21 PF ( they named it PFM ) from the orginal PFM plane. Here in Poland and in Czechoslovakia the PF and PFM names were still in using for instance.
For these who could be interested in the MiG-21 used in the DDR I would recommend the DHS series of books for the plane.
the pic source : the net.
Decided to get the 4 four book and now I got the one on the PF, SPS and SPS K. Can I make a mig 21 PF with GP 9, and FFAR with the JATO boost?
Thanks for your answer, asking because I've seen a pic of a DDRmig 21 no 758 with them.