Is the box 'Green Cardboard' or 'Dark Grey Plastic'
If Green Carboard it is for the original MkI and MkII backs and there is no problem having the film loaded (see image below)
If the plastic box then that is for the newer IIIN backs and as the backs were multi-format (via a masking system) and could also be used for 120 or 220 by changing the insert and the box was designed only for storage, not as a means of having film pre-loaded. A newer IIIN back comes with a pair of inserts one for 120 film one for 220 film and you used the insert required at the time.
The older backs were format and film size dependant so you could buy a back for 120, 220 film and 6x7 as well as 6x8 format (I have a 6x7 as well as x3 6x8 backs) but as far as I know the 120 inserts can be used in any (pre IIIN) 120 back.
Green Boxed insert with Tri X pre-loaded