Pretty sure the change to aluminum hood was 1981. The only model with aluminum trunk lid was 107.026 450SLC5.0 and 500SLC. The one on your 500SL is steel.

Front fenders had the minor change in 1978 when the vacuum adjustable headlights were introduced.

Edit: Remembered a discussion on R107 trunk lids. One person stated his first year 500SL had an aluminum trunk lid. I don't think anyone was ever able to confirm it.