Designs of the OMC crowd was doing a lot of consolidation from 1950 to 1957 That SD model may have been one of the derivatives of the Fastwin, as well as the QD By the mid 50’s, all the twins were one basic design and just different sizes.
I would have to say though, the real parent to the 18, is the 1950 Fastwin 14. 🙂
it is the earlier 19.94 cid block which also the same displacement as the 15 its likely that the SD had some influence on the larger 18 design