A recent date left this for me to find...a surprise gift, how sweet! I'd love to know just how sweet :)
It's 1 3/8 inches, 3.5 cm thick. The larger piece weighs 143 grams or 5 ounces.
It tastes pretty good (but I am definitely no connoisseur) and it's certainly in the semi-sweet/bittersweet range.
I just discovered one more possible hint, WinCo Foods sells chocolate block pieces in their bulk food section, there are WinCos in my date's home state of Idaho. They carry both what they call dark chocolate and bittersweet chocolate.
BUMMER! The bounty has expired, but I will still make good on it. I will give 250 pts to anyone who first positively identifies the mystery chocolate before I can. May the force be with you.