Well, the measuring looks wrong on the second part. From the nut to the 24th fret, your measurement of 18 5/8", matches neither of those scale lengths. For a 24.75" scale length neck, It would be about 18.562"( 18 5/8 is 18.615) from the nut to the 24th and for the 25.5" scale, It would be 19 1/8" exactly to the 24th fret.