Yes, you use a spacer to lower the amount of rounds you can have in the shotgun. This is to abide by hunting laws that deal with limits (how much game you can kill).
I don't know much about Japanese gun laws but meeting a max capacity requirement by way of a spacer that's so trivially removed seems a little strange...
@Kevadu this is actually pretty common, even in the US. Federal law limits ammo capacity to 3 when hunting migratory birds. So it's easier and cheaper to limit your current gun rather than buying a specialized one
I inherited a shotgun (a 1932 Browning) from my father-in-law, and it had a hand-whittled stick in the tube as a spacer, making it only possible to load 2 rounds with one in the chamber.
Since I don't really hunt, I took it out; the tube holds 6, which seems either wild or reassuring, depending upon its intended use at the time of its manufacturing...