Pictured above are the 16K memory select blocks on the motherboard of an Apple II. Since RAM was very expensive in the mid to late 70's  these blocks were removable on early models of the Apple II to allow the user to use the cheaper 4K RAM chips or more expensive 16K RAM chips allowing a wide range of RAM from a low 4 Kilobytes to the maximum 48 Kilobytes of RAM. As the price of RAM dropped the 16K blocks were soldered in place at the factory eliminating the use of the lower density 4 Kilobyte RAM chips on the later models to allow a user to configure the system only as a 16K, 32K, or 48K RAM configuration. Finally as the price of RAM continued to drop and applications required more RAM  most Apple II orders were for 48K systems. The blocks were completely removed on the Apple II+ and the computer could only be ordered in the 48 Kilobyte configuration from the factory