C62 Block Of 4, Error?

In reviewing some of my stamps, I came across the C62 as a block of four with plate# 26970. I noticed, what seems to me, a difference on one the bells. Three of the four are identical, whereas the fourth one looks to have an horizontal extension on the row following the date (MDCCLIII). Curious if this had been noticed previously and if it is, or is not an error. Thanks for helping me out.
See pictures,

There are a couple different types of "errors," each constituting a major mistake whether on the part of the engraver/designer during the initial creation process (a misspelling of a word, the wrong person pictured, etc) or a major mistake that happens during production (like an inverted center/frame, wrong color, missing portion of the design, etc). That being said, I certainly wouldn't categorize your example here as an error. Perhaps a "freak" or "oddity," if anything. If I had to guess, I'd say during printing the "line" you're seeing didn't receive as much ink as the surrounding area, leading to a lighter strip where it usually is dark.

Thank you very much for the explanation. Still learning, and every bit helps.
Thanks again!