If I have a cover with a Scott 10 or Scott 11, and there's a letter inside, and the letter is dated before 1855, and the paper, letter, handwriting, date, and location reasonably matches the postmark and address (i.e. it doesn't seem to be a random letter stuffed into the cover, does it follow that the stamp must also predate 1855 and therefore be Scott 10?
Scott #10/10A were only issued from July to Decmber 1851. There are some usages of #10s after December 1851, but they are relatively scarce. If the letter is dated 1852 or beyond it is more than likely a #11/11A. The only sure way to know would be plate it or if you post a image of the cover we might be able to ID by color.
Scott 10, 10A, 11, and 11A were all used prior to 1855. However, Scott 10 and 10A were almost entirely used before January 1852, though examples do exist used later than that date. If your letter is dated 1855, there is a high probability that it is either a Scott 11 or 11A.
To definitively identify the plate and its position we will need a higher resolution version of the Stamp itself. Having said that, I feel confident that it's not a 10A. Plates 1L, 2L and 3 were all in use in 1853 and all produced 11As. I feel very confident your stamp is from the bottom row of either plate 2L or 3. It's not from 1L because the top right diamond block is not gouged out. Nice cover and stamp.