Bruce,
Many older pools have the main drain plumbed into the bottom of the skimmer. For all practical purposes you do not have a main drain.. Under your skimmer basket, you may have a space ship looking device, called a diverter, that lets the main drain work if the water falls below the level of the skimmer mouth. But I have had zero luck making them work well or consistently.
I have two pools that have the same configuration.. Neither pool has a working main drain and I can't tell any difference in how well they work. They have been that way for at least 10 years.
Pools don't really need a main drains.
Thanks,
Jim R.