The subdirectories are named:

mmmyy [e.g. apr98]  or mmmyyyy [e.g. jan2000]

Individual maps are named:

yymmddhh.gif [e.g. 99122512.gif]

for the analysis (i.e. the best estimate of the state of the atmosphere at that time) and

yymmddhh.f12.gif [e.g. 99122512.f12.gif]
yymmddhh .f24.gif [e.g. 99122512.f24.gif]

for the 12 and 24 hour forecasts valid at that time.