Typically the MB armrests secure to the driver's seat with 2 hex head bolts. A plastic cover snaps in place to hide the bolt heads.

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Fred Hoelzle

my 74 280 has its armrest on the passenger seat same as the 450sel

Regardless, the armrest should fasten to the passenger's seat with 2 hex head bolts. Check your 1974 280 and see how the armrest is fastened.

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Fred Hoelzle