I am deducing from this that the same UPC code is used for the exact same product until the packaging is changed. By "exact same", I mean same manufacturer, same colour, sold within a set time frame. So, if I bought that particular shade, made by that company, even at a different store, but within that time period, the UPC code would be the same as on the receipt found in MTR's home. Therefore, I still think that the date purchased is more significant than the SKU.
But, I don't doubt he purchased both those boxes of hair dye and one was left for TLM.
However, my big question is why would anyone, especially MTR, put the receipt for such inexpensive items in a drawer and keep it for three months? I doubt he was planning to return or exchange them. I keep receipts for items I might have to return and for big-ticket stuff like TVs, appliances, computers, etc. Doesn't make sense to me why he didn't just throw this receipt into the garbage.
JMO