I dropped the same question at Selden (they should know) and they helped me quick.
D stands for Deck stepped, 2 stands for 2-spreaders, 3 stands for fractional rigg, C227 is the mast section. 6417 is a serial number for the specification, SAS is the Selden French subsidiary, 1415 is the production batch.
Hi pbu, That’s good information for all of us who have Selden masts.
By the way, when you were on the Seldenmast website, did you take to time to look at the documents section? Their rigging guide must surely be the best around. And instructions for all the other components and systems are also excellent.