The Tokyo Marui GBB Type 89 folding stock magazine is a shorter 20rd style mag designed to work with said gun, making it a complete paratrooper style rifle.
Compatibility:
Tokyo Marui Type 89 GBBR
Tokyo Marui Type 89 Folding Stock GBBR
Includes:
x1 Tokyo Marui Type 89 Folding Stock 20rds Short Gas Magazine (Compatible with Type 89 J.G.S.D.F)