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Now, can I do the 6913 with these decals?
I think yes, you can. Also both kits are nice buys. However please make sure the PFM kite comes with the side open cockpit canopy and a larger chord fin. Polish PFM Migs were of 94/94A plane type contrary to the similar but not used by the PAF ( the forward open solid cockpit canopy ) plane type 77.
Also pay your attention to that although the 6913 was a PFM variant she got later the cockpit canopy with the periscope at the hood top. While the shot of her at the early service shows this without that one.
No excuses now Jan, get to it. i must say the Mig 21PF is a real nice kit.