Let's say, you're heading to the store...
Someone gives you cash, and they ask you to pick up a pack of M&Ms. You get to the store and the store has plan and peanut M&Ms. (You do NOT have the option to call for clarification, and you MUST buy one of the M&Ms.)
Which M&Ms do you buy?
I'm eating a bag of M&Ms right now, and I'm reminded of a conversation I had a looong time ago. It was a friendly debate about which M&Ms (plain or peanut) are the default/standard kind of M&Ms.
IMO, peanut M&Ms are the default, and that's what I would buy. I would never[i] assume plain M&Ms, unless they are specified.