Phil - I think if you look carefully at the headings to each of these Spanish Civil War headstamps on Willems’s list, you will find he does show what the letters mean:
The Roman numerals identify which activity falling under the various companys’ sphere of influence, the round was made at. For example, let’s take Polte (B):
I: P (Werk Magdeburg)
The other numbers are basically the lot number and the year.
So, your box label you show is from the major Polte company, but with case made and loaded in the factory Silva Metallwerke G.m.b.H., at Genthin.
The only thing is don’t know from this is the exact relationship that “Parent Company” Polte had with the various factories using their headstamp “B” with the various Roman Numerals - outside ownership, war-time stewardship simply assigned by the Reich, major financial interest thru stock control, etc. With DWM and RWS, it is obvious. The factories represented by the Roman numerals are, in the case of those two, facilities of the parent company operated under DWM or RWS’s actual name (P120 was Dynamit A.-G., but they were the parent company for RWS and Geco, at least).