Capitol 930 Series Patio sliding Door Roller Assembly, Flat 1inch Diameter Steel Wheel. 
Used on old capitol doors, maybe the 930 series. 

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1 inch flat steel ball bearing roller unit
Adjustable steel housing assembly

Diameter: 1-inch
Housing Tab Style: Plain Back
Housing Width: 19/32-inch
Material: Steel
Roller Diameter: 1-inch
Roller Edge Type: Flat
Roller Material: Steel
Roller Width: 5/16-inch
Type: Flat
Width: 19/32-inch

Helpful Article: How to Replace Rollers in an Aluminum Sliding Glass Door