This instructable is mostly concerned with the method(s) of working in scale from photograph to final item.
Step 1: Collecting and collating reference material
Fortunately for us, however, they all hewed to a regulation size; 40 mm x 10 mm. These round numbers made working to scale much easier.
Step 2: Scaling reference material
The size of the grid is actually arbitrary. All I needed was a grid that contained 4 horizontal divisions the images could be scaled to. Oddly enough, setting the grid to 1 cm turned out to be that size. I then scaled the layer containing the clasps until they exactly lined up with the grid.
The last step was to crop the image at, again, a known value and at an exact square ratio. I chose to give myself 1 cm borders for ease in the modeling application; thus the final save was a square image of arbitrary pixel size that represented a 6 cm by 6 cm square in scale.