Lightdoor on APS cartriges

I am trying to load APS film into my Box camera[1] (for 120 roll
film) and I am wondering how the light door on the APS cartridge
works:

When I open the door (by pulling the small lever in the hole right
next to the door on the top of the cartridge, will *all* the film
in the cartridge be exposed to light, or will (just like with 35mm
cartridges) only the part outside the cartridge be affected.

In other words: Is there a further light seal inside the cartridge
beside the small "door" that can be opened with a small screwdriver,
or is this the only protection.

If there is a further light seal, I could open the cartridge,
pull out a piece of film and put it into my box camera in normal
daylight, otherwise I'll have to do it in the darkroom (much more
error prone and difficult).

TIA
/ralph

[1] no joke, I'm just experimenting, like others are with their
Holgas. Box cameras are much cheaper than Holgas here in Austria
though.
--
The truth can't hurt you it's just like the dark
It scares you witless
But in time you see things clear and stark
Elvis Costello