There were changes in relays during this time: some 4 prong, some 5 prong, some fast glow, etc. It's not so much whether the relay is for a 240 or 300, it is more the type of glow plugs, etc. Take your original relay and try to match it at the salvage yard or, if you are sure it is the relay you can always spend the $120ish to buy a new replacement.

All this being said, I am sure someone with a more exacting recommendation will chime in shortly.

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too many w123's
84 280 SL 5 speed