Select epoch prefix 'B' or 'J'.
     jb     int         slaDbjin prefix status:  0=none, 1='B', 2='J'
     e      double      epoch - Besselian or Julian
     *k     char        'B' or 'J'
     *j     int         status:  0=OK
  If jb=0, B is assumed for e < 1984.0, otherwise J.
  P.T.Wallace   Starlink   23 October 1993