On the left is a US shoulder holster that was acquired along with an Ithaca WWII era 1911 auto pistol. The holster was made by Enger-Kress in 1944.
This type of holster was used from WWII through Vietnam by pilots, aircrews, and tankers. Snapped leather safety strap, adjustable leather harness, and leather belt hold-down loop. Stamped "US" on front of holster. Strap may be worn under the right arm and across the chest or over the right shoulder and behind the back.
The item on the right is a fascinating one. In my travels, I have hardly ever come across any of these slides. Notice on the one on the right that the handwriting states, "2nd Exam".