Andy,
I'm fluent in Michel, so fire away with any questions. First, if you don't have it yet, download their
guide for English speakers (1MB PDF).
Michel's basic number/letter system is as follows (quoting from their English-language catalogs):
PrefixesD = Official stamp
H-Bl. = Stamp Booklet page
Hz = Center piece (block of 4 to 9) of a booklet sheet
K = Tete-beche (from stamp booklet sheet)
KZ = Tete-beche with gutter (from stamp booklet sheet)
MH = Stamp Booklet
MHB = Stamp Booklet sheet
P = Postage Due stamp
S = vertical se-tenant(from stamp booklet sheet)
SZ = vertical se-tenant with gutter (from stamp booklet sheet)
SZd = vertical se-tenant (from counter sheet)
W = horizontal se-tenant (from stamp booklet sheet)
WZ = horizontal se-tenant with gutter (from stamp booklet sheet)
WZd = horizontal se-tenant (from counter sheet)
SuffixesA, B, C, D, E... (the first upper case letters of the alphabet) after the number indicate the perforation type of the stamp.
a, b, c, d, e... (the first lower case letters of the alphabet) after the number indicate important color differences.
...V, W, X, Y, Z (the last upper case letters of the alphabet) after the number indicate the watermark varieties.
...v, w, x, y, z (the last lower case letters of the alphabet) after the number indicate differences in paper and gum.
I, II, III, IV, V... (Roman numerals after Arabic numerals) indicate printing and type differences.
L = Empty field in same size as stamp
P = Flat Plate printing (sheet printing) or Proof
R = Coil stamp
W = Rotary Press printing (Roller printing)
Zf = empty spaces (sometimes with ornamentation) in same size as stamp
ZS = vertical gutter pair
ZW = horizontal gutter pair
DD = double (-image) -print or double overprint
DK = double overprint, one inverted
F = printing error or color error
G = printed on the gummed side
K = Tete-beche or inverted overprint
Pa = on glued paper web (two pieces of paper glued together to form continuous printing process)
U = imperforate (subtypes indicate which sides, if less than all four - ex. Ur = imperf at right)
I, II, III, IV, V... (Roman numerals at the end of the MICHEL Number) can also be used to indicate plate flaws.
Stamps that were not issued for whatever reason, receive Roman numerals without a Michel number preceding and without suffixes.
For example:
Bavaria MiNr. 25 X b U: 25 (major listing number) X (watermark) b (color) you (imperforate).