There is a ball-filter (a bit like a golf ball) fitted in the inside to block tea leaves from leaving the sprout when pouring tea. The teapot was designed this way for the Japanese market in the 70s to early 80s. As this teapot design roughly indicates the era it was made, there are imitation ball-filter teapots around.
As ball-filter teapots were made more than 30 years ago, they were made using the original fine Yixing clay with good workmanship. Occasionally, the teapots could be made by craftsmasters or even grandmasters of Yixing. I have several ball-filter teapots made by craftsmasters during their younger years at the Yixing factory. There is so much fun collecting teapots. You may bump into some really good one at affordable prices. However, one has to be extra careful not to taken for a ride as there are just too many fakes posing as genuine ones.