Are the 2 round knobs on the dash of a 1971 Type 3 for the shut off of fresh air to the windscreen?

If so mine dont seem to shut off so what might be the problem?

Yep, they also control the air to the two round vents on the dash.. One does the left hand side and the other does the right hand side.

The knobs open and close flaps inside the fresh air box under the dash via cables. It is possible that the cables have snapped or slipped out of their clamps on the fresh air box. The only way to check is to pull the box out and have a look. It is also very common for the fresh air box to get filled with leaves and crap, so it's worthwhile so you can clean it out. It is a fiddly job which requires a couple of hours bent in a very uncomfortable position under the dash.