I believe there's essentially 3 major versions of the prop.
V1: The original used a modified tape measure holster, this is the version with the Sanyo Calculator. This one looks very clean and has a boxy structure to it. The community has dubbed this the 'hero' prop but it's actually seen very little in the movie. From what I can tell it was only used early on for the exterior New York shoot, and was seemingly abandoned thereafter.
V2: The prop primarily used in the first film used a simple scratch built holster. Note that the outer strip of leather folds around the circuit board before turning back out to be riveted in place. This secures the board and pushes it out a bit from the leather backing.
V3: The GB2 holster, this was constructed identically to V2, but for whatever reason they didn't take the time to shape the outer loop so that it wraps around the circuit board. This leaves the loop sticking out quite a bit which is silly imo.