you are correct! The only NRA sanctioned F-Class matches I ever shot were at Bogue Chitto and Pascagoula. The last year I shot at each they started doing the Convertibles, they have been in the rule book since 2010. If my memory serves me right, I think we did 5 sighters on the first string of 20 and 2 for the second two strings of 20. No you cannot just choose an X out of your the sighters. The way the rule reads (and Roland, Mr. F-Class can clarify) You can take sighter 1 & 2 or sighter 2. During the second string where only two are allowed, you can take the 1st and 2nd sighter or you can take only the 2nd. If Reese Bottom gives 7 sighters on the first round it would not be an issue. Shooter tells the Scorer when to start recording. If seven sighters are taken and the shooter wants to convert, he can have number 7 or 6 & 7. But there again you cannot chose amongst all of the sighters, just the last or the last two.
Baron Graham IBS 2010