Tongue-and-groove is called "pontti" in Finnish. And the type I'm using here is called "puolipontti", where "puoli" means like "semi-" or "half-". So, there is not an actual groove, only the lower side of each panel is cut thinner, and the upper side has a tongue. Maybe I can post a detailed picture as a daily picture later on.

These are the panels mentioned in the earlier post: http://www.enormouselk.com/?q=erkkasblog/plan-and-flow