I'm not as up on the 603 as I could be but that looks like your switchover valve to me. It cuts fuel if the turbo overboosts (when it is hooked up anyway).

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine