It is still in the Gov's database, it's new number is "5120-00-227-6693", the fact it was missing the -00- means it is old enough to predate the last revision where they added more digts (but it's post war, as it says 'FSN')
The database says an exception was made for it to have some non standard configuration for a special purpose, but does not say what it fits. I can' find anything that seems to need it...could be something classified tho ....(There is tons of stuff that's classified just because noone cared enough to unclassify it)