This is from an interview with Shogo Hasui. Source: http://news.tfw2005.com/2012/03/26/t...60#more-174660

– How were the accessories selected?
Hasui: The gimmicks and accessories that were present with MP-1 were omitted as far as possible because I felt doing the same was not interesting. I tried to include as many new gimmicks as the cost allowed. The Spike figure, the gun storage and Roller were the result of this.
– Was the gun storage in his back inspired by the American comics?
Hasui: There was certainly an influence from the American comics, but basically that is taken from the cartoon, too. In many scenes, Convoy transforms with his gun. I wanted to make the most of that image. I assumed Convoy had a system to transform with his gun and incorporated (the gimmick).