Which set is this 2 x 6 slope with holes from?
You have a Fabuland Roof Support Slope, 6 x 2.



If it's like the similar ones I've seen in Duplo, it probably came pre-assembled into one of these:

Fabuland Roof Support with Gray Roof Slope without Chimney Hole



Fabuland Roof Support with Red Roof Slope with Chimney Hole



Fabuland Roof Support with Red Roof Slope without Chimney Hole