Scott 515 Different Foot Of Left 2 In 20

Foot of left 2 in 20 is significantly different from the right or from the left one in another stamp.

Probably just a printing oddity, but looks interesting. Any ideas?

Both stamps are 515, perforated 11.

Thanks.

I've seen all kinds of oddities in all types of stamps. Once you start looking closely you start noticing things. Thought someone may like this

It is surprising there is as much consistency as there is, considering how many 100s of millions of some of these stamps were printed over a span of years. Consider the moving parts:
The paper: slightly dampened to optimally accept the ink. All sheets damped exactly the same from middle to edge to corner?
The plate: From brand new to worn condition.
The ink: a fresh batch or the bottom of the barrel? Stirred frequently?
The press. Same, similar, or different?
The room: warmer/colder, dry/humid, Monday/Friday, etc.
The personnel: Diligent or distracted? Experienced or the summer intern?
Etc.
I note the area circled in red .... differing from the same area on the left stamp.

I still fail to see any difference in the black oval highlighted in the image...
.

The paper: slightly dampened to optimally accept the ink. All sheets damped exactly the same from middle to edge to corner?
Pretty much yes. The evening before press day, the paper sheets were stacked between damp cloths and stayed in a closed container over night. This ensured evenly wetted sheets. Not "slightly wetted" but pretty much wet.

The plate: From brand new to worn condition.
Main reason

The ink: a fresh batch or the bottom of the barrel? Stirred frequently?
No stirring, might have been occasionally refilled ink trays when required during printing

The press. Same, similar, or different?
Four plate power presses for the W/F, identical

The room: warmer/colder, dry/humid, Monday/Friday, etc.
Mostly irrelevant. Only the gum was the problem, hence there were three different gum solutions for winter, summer and spring/fall.

The personnel: Diligent or distracted? Experienced or the summer intern?
Experienced. These stamps were produced on early versions of the 4ppp operated by three people. These were experienced workers (one pressman, two "girls" as they were called at the time. Any larger mistakes would result in printer's waste.

The main cause of inking differences is plate wear. As the plate gets thinner, the grooves and pits get shallower over time. This results in lines getting thinner and/or dots diappearing due to ink scooping and less ink deposited in the grooves.

In the image, the right stamp looks somewhat out of focus giving it a fuzzier look than the left stamp. The red oval shows a case of likely significant plate wear The horizontal grooves had become so shallow that almost no ink remained in the grooves and the whole thing would essentially become an uninked area in the end.

Well, I mostly work with worldwide stamps, a lot of British Commonwealth. Variations like these don't happen quite as often on those stamps. But I see your point. In fact, the picture below (borrowed from HipStamp) shows variation between nearly every 2 in 20 just in this block alone. If this was an early GB block, SG would have assigned a different catalog number for each.

I believe the quantities produced of these US workhorses were many multiples of the highest production BC issues thus the wear issues.

No. And they are plenty as many stamps were printed over long periods of time. It would be listed only if it was a constant variety (die error, transfer error, plate damage) and of interest. Since wear is progressive, the variety is likely to be transient. SG does not list wear varieties unless they are very major and collectable. When they do, it tends to be a 'shade' variety attributable to plate wear. A minor wear variety that does not jump out at you is not listed.