Short version first: Sp is single propeller. One propeller. Duoprop DP came in 1983 and replaced single plug. The front shell between 280 and 290 first version is mostly that 280 can not be trimmed hydraulically but it can 290. Otherwise they are the same whether it is SP or DP.
The 290 drive can largely be divided into two generation openings
First generation from 1985 to April 1991.: 18mm suspension bolts. Springs and hooks that keep the drive down when reversing. (too much clay, easily affected by shells / growth) In addition, this has driven 3.5 inch exhaust pipe light gray-white color.
Second generation: from April 1991: 25mm suspension bolts, Springs and hooks are gone. The exhaust pipe inside is 4 inches and has a green color.