FSX. SkySimulations is honestly an extremely quiet developer and it's always hard to determine if these guys still exist or not. They really do not appear to have much of an outgoing open social relationship with their customers the way many other developers do in terms of forums and social media. People really like to know what's going on with the development community. That said, it seems SkySimulations is indeed still alive and well with an all new version of their DC-9 which includes far too much to list here. No word about P3D compatibility. My current one works in P3D but not sure if this one will.
UPDATE OFFER: If you previously purchased SKY SIMULATIONS - MCDONNELL DOUGLAS DC-9 FSX at simMarket, you are entitled to the special upgrade price of EUR 18.00 only. NOTE: The special upgrade price will be awarded automatically, therefore you must be logged into your same account used to purchase the previous version! If in doubt contact simMarket support first! NO REFUNDS GIVEN! More information available via the source link below. Thanks to ADX pal Jordan King (of Jordan King Productions) for the tipoff.