That looks like a Schwinn middleweight cantilever frame. The model names varied through the years. According to the Schwinn serial number lookup it was manufactured in Chicago in December of 1966. It could have been a 1966 model shipped and delivered just in time for Christmas or it could have been a 1967 model.

By checking the catalogs for 1966 and 1967, and looking for a Sky Blue cantilever frame, it could have been a Deluxe Typhoon or a Panther.

By the way, the other quick way to tell that it is a Schwinn is that it has the kickstand built in.

Neat bike. The chainguard and the front sprocket are not original. But the crank looks to be an original Schwinn crank.

You should be able to find a Schwinn seat post clamp on eBay or at if you have a bike coop near you they can be great resources of parts.

Here is one on Ebay: Schwinn Seat Post Clamp