Franklin B. Mallory (The Krag Rifle Story) and William S. Brophy (The Krag Rifle & The 1903 Springfield Rifle) used photos of various early 1900's Frankford Reloading Tools in their books. They both show the '1905 Set', also shown, in the Link posted by 'Local Boy'.

While not specifically identified, I believe the dual purpose 'plate' was a gauge used to check certain aspects of reloaded .30-40 and .30-03 cartridges.

Could "J.M.G." stand for something like, "Jacketed Munitions Gauge"?