The Sony Walkman WM 40 also known as WM 10 RV was the last of the Sony Walkmans that had to be expanded to accept a cassette. A completely new internal mechanism and cassette door was designed for this Walkman. The WM 40 came in four colors. The WM 10 RV only came in the silver/gold color unless I'm mistaken. The WM 40's were also able to play AM/FM radio. It needed a special radio tuner pack called CTP-1A shaped like a cassette. Insert it, press play and you can enjoy radio in AM/FM. The radio tuner pack is also compatible with the WM-55 and WM-W800 players. In all the years of collecting, the blue WM 40 is the rarest. Was lucky to purchase one from Japan and so far I have not seen another collector with one.