I contacted the seller and received this message in reply:
"
You bought the wrong memory for your Apple Macintosh computers. Both of them (LC 575 and a Performa 6300CD) can take a max of 32MB simm made of two memory banks. These 64MB simms we sold are 64MB made of a single memory bank. Each of your Apple Macintosh computers can read up to two memory banks at 16MB each. Therefore, it can only read 16MB out of the total 64MB simms.
"
Now I KNOW that the Performa can accept 64MB modules, and the LC should too. Or does he know something that I don't know?
"
You bought the wrong memory for your Apple Macintosh computers. Both of them (LC 575 and a Performa 6300CD) can take a max of 32MB simm made of two memory banks. These 64MB simms we sold are 64MB made of a single memory bank. Each of your Apple Macintosh computers can read up to two memory banks at 16MB each. Therefore, it can only read 16MB out of the total 64MB simms.
"
Now I KNOW that the Performa can accept 64MB modules, and the LC should too. Or does he know something that I don't know?