Two guesses..
900SL wroteAny idea as to why one leg (lhs) is stamped Ducati yet the rhs leg has a casting mark with no logo?
Is it a later production item, post the font change for the Ducati logo (not long before the 900 FE was released) ?
Or could it be a boss for locating the orange plastic reflector that some markets specified. I've seen them positioned alternately on either the fork stanchion or in between the numberplate and rear indicators, on the side of the integral undertray/ mud guard (that often gets cut-off).
Phil