Hi Harper (and any others who may have an interest) --
Here is some information about how to identify the A through F reliefs on the 1857 - 1861 3c perforated issue.
First up is the original email from Dick Celler discussing his procedure for figuring the relief. This is pretty much a repeat of what txstamp has already posted -- but I post it here anyway as Dick's email is relative to his plating mats.
Next up are copies of the hand drawn plating mats that Dick made up back in 1997. There is a separate mat for each relief. I wish I had better quality images of these mats -- as it is sometimes a bit difficult to see where Dick highlighted the mat -- but alas -- what I am posting here is the best I have. And thanks again to Don for hosting full sized images on all of these files enabling their posting here on SCF.
Following the plating mats, there are examples of actual stamps for each relief. There are 9 examples shown -- as for the A relief -- there are 2 examples -- one without, and one with, the repair to the damaged area of the transfer role. Also, for the F relief there is an example of a 6th row F (i.e., F6) and a 10th row F (i.e., F10) -- as there are very subtle differences between the two.
As an aside, there's only one place in the 6-relief 3c plates that I can think of where there are misplaced reliefs -- which would be 70-80-90-100R11i. Instead of C-D-E-F when they reentered the plate, they used B-C-D-E, and wound up with the top of an 11th row stamp in the bottom margin. As a bonus scan -- I am including a high resolution scan of the famous misplaced E relief that was pictured in the Sesqui book -- (i.e., 100R11i -- the stamp with the 11th row).
Lastly -- For purposes of full disclosure -- I do not own any of the stamps shown below. All of these images were given to me by Dick Celler back when he was teaching me how to ID the various reliefs.
Regards // ioagoa
Here is Dick's email on relief ID -- which, as txstamp noted, he (Dick) sent to a number of folks who had an interest in learning how to ID the A through F reliefs on the 1857 - 1861 3c perforated issue:

Next up are the 6 plating mats -- one for each relief (with the applicable relief being noted in upper left corner of each mat) --






And here are actual stamps showing an example of each relief:
A relief without repair -- position 9L10E
-9L10E%20CC.jpg)
A relief with repair -- position 1L11E
-1L11E%20TJA.jpg)
B relief -- position 15L11E

C relief -- position 25L11E

D relief -- position 77R11E

E relief -- misplaced -- 100R11i -- (showing the 11th row effect as discussed in the Sesqui book)
-100R11i.jpg)
E relief -- position 45R11E

F6 relief (i.e., 6th row F) -- position 58R11E (showing a row 6 guide dot)
-58R11E%20KG.jpg)
F10 relief (i.e., 10th row F) -- position 92R11E
-92R11E%20CDC.jpg)