Not sure you'll get many responses here because most users haven't been brazen enough to take the back cover off their phone thus voiding the warranty etc. I'll offer a SWAG and say those letters/numbers are part of a manufacturing process that identifies assemblers or date/time/batch/location that the phone was assembled...just a guess.
And likely a very good guess! During the manufacturing process, [virtually all parts, even] small parts such as these go through several processes from the intial molding of the plastic surround to de-burring, drilling and other additional manufacturing processes, to adhering of the Kevlar, and to final surface preparations before assembly. Each of these processes is often checked by either the individual running the equipment or a QC personnel somewhere further down the assembly line.
As each part is further "dressed" such as to remove small burrs or other extra material from the molding process, often the person running the equipment will mark it to assure someone further down the line that it passes their visual exam. You are likely to get no benefit from knowing those marks other than what's been explained.
I will remind you however that by opening the case you have voided your warranty, you have introduced dust and other contaminants into the interior of the phone, you may have damaged the airtight seal that exists around the main microphone and also around the speaker and noise cancelling microphone to create a small hollow chamber (similar to an acoustically sealed speaker cabinet), and enhance audio quality. You could also have damaged the rear-facing camera which is a highly delicate instrument due to its miniscule size and critical alignments.
You may have damaged or compromised the lamination benefits of the double-sided tape that holds the back and battery together - reducing the ability to withstand flexing, and you may also have damaged the water-resistant surface applications that exist to protect the phone's circuitry. There are also very delicate ribbon cables (including the one which connects the battery), and snap fit connectors throughout which can only withstand very minor movement or pressures, and even the slightest touch could render one of them unserviceable.
I suggest you put the covers back on and use the phones as intended, and resist digging further into them as you may render them useless.
