I've never heard the term "Frost Plugs", so I looked it up. Here is what I found - "Core plugs are used to fill the sand casting core holes found on water-cooled internal combustion engines. They are also commonly called frost plugs, freeze plugs, or engine block expansion plugs". I grew up in the midwest and we called them Freeze Plugs.
What I do know is that the terms Frost and Freeze plugs are a misnomer, They will not push out if the water freezes in the block. The block will crack if that happens. This might be a regional term but Core Plugs tell you what they are in the name the way most parts are named. Just an FYI.