Patio door roller.
A handy person from Vista, CA says:
June 17, 2015
Part is 3 1/4 long, 1 3/4 tall, 5/8 thick. There are about 10 small cylindrical rollers in the narrow view picture. They roll around a "landing" in the housing which transfers the weight of the door to the rollers. This roller assembly was used by "Thermoview" on their heavy patio doors. I could not find any part number or other identifying marks on the assembly.
Paul from SWISCO responded:
June 18, 2015
This looks like our 81-280, but your height is different than ours. Please double check your measurements to confirm.