Sometime in the 1960s, the Japanese changed how they made backs. Instead of using a labyrinth design, they used foam to block light. While effective, the foam has a finite life and will need to be replaced.
Today, nearly all Japanese cameras (in truth, all), from the 1960s and 1970s will need to be re-foamed. There are inexpensive kits available on eBay. Or you can buy the material yourself and cut your own foam using a hobby knife. If you have many cameras to re-foam, it's less expensive to buy your own. What you want is a material from a craft shop, such as Michael's (in the U.S.A.). I've used the Foamies brand, which is sold in 9x11 sheets with self-adhesive backing.
European cameras use a different back design and will rarely have foam to block light.
It's simply a different approach. Think of how U.S., Japanese and European car makers produce vehicles for the luxury, performance and touring segments. Which is best? It depends on whom you ask.
When you replace your foam, I would recommend lighter fluid to help remove all traces of the old foam. Be careful to not allow foam to get into the shutter mechanism and especially don't let it get on an SLR's viewing screen.