Task: Question Understanding
Input (English): Context : Alternate names include Ruhrstahl SD 1400 X, Kramer X-1, PC 1400X or FX 1400 (the latter, along with the unguided PC 1400 ordnance's Fritz nickname, is the origin for the name ``Fritz X''). 
Question : What year did Fritz X come into use?
Output (English): service entry