Dan,
I no longer collect these. Unlike meter stamps, I know of no general/specialized catalog that lists these with decent coverage. I can only make the following suggestions:
1. Take a quick look at how Scott categorizes/lists the officials for Argentina and Great Britain, and also how Scott lists booklet/pane varieties/slogans (see US Specialized). That's the basis for my suggestions.
2. First make a list of the basic emblem types, ignoring uniform scaling and thickness issues. For example: basic emblem, wide emblem, emblem with trademark... and assign a design letter to it (A, B, C...)
3. Disregard department ID and make a list of basic "slogans/inscriptions" such as: POSTAGE PAID, POSTAGE AND FEES PAID, bar-code, inscription combined with barcode... and assign a lower case Roman numeral (i, ii, iii...)
4. Separate the covers according to department usage. For each department, you can assign your own ID number for each different cover (1, 2, 3...) based on departmental inscription.
5. For each department, each cover will have a catalog of the form (design letter)(slogan number)(ID number)(minor variety), where minor variety is a lower case letter designating varieties such as ink, font, line thickness in design, cover size/paper/color...
So, your ID number for each department would look something like Bii3a. You don't have to choose the minor variety letter alphabetically. You can choose a letter that most easily helps you remember the variety.
Anyway, you don't have to follow my suggestions exactly. It's just to give you an idea of how you might want to proceed.
Enjoy your covers!

k