I want to get a master cylinder for my 85 300SD they seem to go my chassis numbers where do I find that number?

Individual chassis sequential number is the last six digits of the VIN.

Thank You

Don't know about the SD but in a W123, chassis number is stamped into the top/ middle of the firewall in the engine compartment.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked