Writing or phoning defintely the best way of visiting (need to complete security proformas prior to your visit in most cases), or knowing somebody at a given unit!
As per the FPL Q?......
Field 9 of the Flight Plan:
B744 = Aircraft Type (Boeing 747-400)
H = Heavy
Field 10 of the Flight Plan:
S = standard COM/NAV/Approach Aid equipment for the route to be flown is caried and serviceable (Standard equipment is VHF RT, ADF, VOR ILS)
X = MNPS Approved (minimum nav performance specifications)
W = RVSM approved (Reduced Vertical Seperation Minima)
I = Inertial Navigation
Y = Radio with 8.33 KHz spacing
C = Transponder Mode A 4096 codes and mode Charlie
......I think!!!