My 1600 series has removable screens, so assuming yours are missing. They should slide into the upper channel and settle into the lower channel. 2 thicknesses of screen frame commonly available at home centers. It's the thinner size. Can't recall the exact dimension.
By the way, the screens depend on the age of Hehr window. Older ones (40s-early 60s) used the screen grooves and opened with a notched handle. The later Hehrs were often Jalousies and had the slip up and in screens.
Every major hardware chain in the States sells mill finish frames. Plenty of online sources too, if that's an option. Should be the 5/16" size. If you want the vintage look, get the kits that allow for mitered corners. Never liked those plastic squares. FYI, the light or medium gray screen material looks the best.