A short article speculating on the origin of a distinctive reverse error on Edward’s class 1c pennies. The author would be interested in hearing from anyone with further examples.
A Peculiar Series of Reverse Die Cutting Errors or Privy Marking of Edward I Class 1c Pennies
BNS Research Blog 
I was interested to read Mr. Greenhalgh’s article pointing out the “drifting” in the pellets of the inner circle on some of the reverse dies of the Edward I 1c coins. His insights on matters of die cutting always provide a valuable contribution to the study of hammered coins.
His suggestion that this feature could have provided a means of privy marking is an intriguing one. If such markings had been deemed necessary, however, surely the use of normal or unbarred N’s on the obverse and reverse giving a total of 16 possible combinations for any one coin would have been sufficient. Add to this the not uncommon appearance of a pellet after DNS on the obverse many such coins would increase this number to 32. This would provide an ample number of possible privy markings to cover a period of a few months in a more acceptably artistic way than in a peculiarity of the inner circles.
A similar use of forms of the letter N as a privy mark was suggested as a possibility by J Shirley-Fox in his ground breaking article on Edward III’s Florin coinage in the Numismatic Chronicle, Vol. 8, 1928.
In my comment above my apologies for writing “unbarred N’s” when I meant “reversed N’s”
I certainly agree with Dennis for the main output of dies. Hence my question that these dies may have originated away from the central Tower die cutting shop. Perhaps the term privy marking may be mistaken for the Pyx marking, that would be extensively used at the Tower and subsequent provincial mints, I should probably have used the term private die-cutting marks