Offline validator for an Austrian Firmenbuchnummer (FN), the company register number. Normalizes the numeric part (1-6 digits, left-padded to 6 for calculation), computes the modulo-11 check letter using weights 2,1,2,1,2,1 (Luhn-style digit reduction for products >=9 by subtracting 9) and the standard letter table (0=p,1=a,2=b,3=c,4=d,5=e,6=f,7=g,8=h,9=i,10=k), and compares it to the supplied check letter.

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