Oh dear, I'm sad enough (and way old enough) to know this stuff.....
The letters do indeed have a meaning - you do need to check out the leaflets to get the details but yes, the first letter gives the working area of the inventory; A for Admin for example, which is why all Barrack Block Inventories generally begin with A (and, incidentally Supply Sqn Inventories do as well because Supply Sqdns in the UK used to be part of Admin Wing.... but that's another sandbag). B is air engineering, C Ground Eng (ing MT), J for Parented units etc. Within each 1st letter, the second letter also has a defined meaning - e.g. A being headquarters. The third is free to use so, in theory, there could be up to 25 inventories in any given Area/sub-area - e.g. Admin Headquarters AAA - AAY.
Over the 40 years since the system was defined most units have had to deviate from it to some extent because of formation changes etc so the pattern is less obvious now but it still holds good at most units.
Beyond that, the Weapon System Code is indeed still 2 letters derived from the Main Equipment Type (MET) Code which is in turn associated with the Type & Mark Applicability TMA codes in the background and allows some limited linkage of transactions with the costs of usage.
The final, numeric digit is the Cost Code and was a early attempt to allow costs of transactions to be linked to specific activities - e.g. Op Fresco etc. The codes were controlled by the FLCs, but these days there are other ways of capturing costs.
)