Looking in the 250 SL book, i see two possibles (which I am typing as listed in the parts book)
DIN 007981004331, lens head screw, sheet metal. The DIN reference book gives BZ 4.2 X16 DBL 8451-21 (high polish galvanic iridescent passivation,standard version, which would be a 4.2 mm diameter, 16 mm length
or
DIN 007981 003228 lens head screw, sheet metal (mounting angle to ornamental frame) this would be a BZ 3.9 mm diameter, 9.5 mm length, with the same finish DBL8451-21.
Verification?