The Performa all in ones are:

Performa 200 aka Classic II

Performa 250 aka Colour Classic

Performa 275 aka Colour Classic II

Performa 5xx aka LC 520, 550, 575, 580, etc

Performa 5xxx aka Power Mac 5xxx

If it was PowerPC Performa, it must have been an upgraded 5xx series or a 5xxx series. If it wasn't necessarily a Performa and those case designs don't match, perhaps it was a Power Mac G3 AIO?