Unlike the plate dots which were intentionally added to the plate for guiding the placement of the guide lines,  many accidental or randomly placed , marks may be found on the Penny Blacks.  These are often small and would be insignificant to the philatelist if not for the fact that they are sometimes constant varieties or flaws. If the mark is a constant variety or flaw, and always appears on a specific stamp  from a specific plate, it serves as a means of identifying the stamp.  The most obvious marks that come to mind are the "Ray Flaws," The "O" Flaw, and the "O Bulge" which make identification of a number of the Black Plates easy.  Many of these marks are small and can go easily unnoticed.  Some examples are provided below:
 
 

Examples Taken From Plate 1b

AL: Diagonal Scratch	SI: Scratch on "OS" of "POSTAGE"

These marks may be the single most important element in the identification of the stamp, in the case of plates that do not have any
high profile flaws or characteristics, or they may contribute to the confirmation of the correct plate of the stamp.  In this case, the
above examples are from Plate 1b. On stamp AL we can note that the check letter "L" is placed close to the left and rather high in
the check letter square.  We can also note the characteristic absence of the 10 O'clock ray in the north-west star.  These three
separate elements together make the identification of the plate conclusive.