I'll always remember what we had
I love you.
The separated pendants reflects the imminent end of their relationship. It is safe to assume they actually have these pendants, as there is a receipt from a jewelry store with engraving instructions, although those instructions don't quite match the engraving on the drawings.
Sam and Lonnie have spent the day around the house enjoying their last day together, as Lonnie is committed to the Army and is leaving for basic training the next day. Sam (perhaps) drew the pendants broken apart and, after Sam falls asleep in Lonnie's arms, Lonnie finds it, adds her note to reassure Sam and leaves it for her before leaving for the bus.
There appears to be erased words behind Lonnie's signature, starting with the word "Go" and ending in what looks like a lower-case "e" near the "i" in "Lonnie." Possibly Lonnie had written "goodbye" then erased it.