Surviving frame numbers range from MF466 to 23031, and glancing through the LDMCC machine register I do not see any obvious one thousand wide gaps indicating they skipped a big block of numbers. There is no evidence anywhere else Douglas resorted to that trick to inflate numbers, like some other vehicle manufactures did.
The definitive 1927 feature is the transmission clamping to the frame tubes, rather than studs through a platform. At what frame number these started, I don't know. Douglas did not change the frame prefix code, but kept on with the old one. The MF12000 number in the model ID guide is an approximation based on the LDMCC Registry. While there are some reported as 1927 models prior to that mixed in with the 1926 machines, at that number the date changes pretty much solidly to 1927. Again, there are some exceptions; and it is dependent on the owner having correctly reported the date.
For engines it is even more murky! The single pipe oil system barely made it out of late 1925, as is my understanding. The majority were retrofitted with the 2-pipe oil system under warranty. Any engine after YE6500, or YE6417, according to source, had the 2-pipe system. So yours is far into the later oiling system. It sounds like your engine, or at least the crankcases, are c1928-29. The A29 being the last year for the 350EW in flat tank guise. No reason why that engine could not be in an earlier, 1926, frame. The frame number is on the saddle lug, right-hand side, on the lug for the down tube.
The detachable spline brake drum was introduced on the B28 and C28 models, where as the A28 (and A29) continued on with the old non-detectable drums. The wheel assemblies were interchangeable.
-Doug
