The polycontrast filter box is not a Beseler product, but an addition from another source. It looks like it is "press fitted", meaning that the box will pop apart if leverage is applied to metal joining edges. Experiment a little, for as it is the unit is useless.
The small filter drawer came factory with a red filter in it. It fits the now long discontinued Beseler polycontrast filter set, and IIRC, a set that Dupont made for their paper line. Both are perfectly obsolete now, as papers have changed and the original filters that you might find on fleabay are usually faded. Your system of placing the Ilfords is the best course.
Most of the parts for the head are still available from Beseler. I also have the hard-to-find alignment instructions for this head. If interested, PM me.