Here's all of the modern hollowgram arms.
Cannondale Si (solid forged)
Hollowgram Si (one piece, hollow)
SiSL2 (two piece bonded hollow)
I believe the SL and old silver SI can work with the modern spindles, the length of the spindles could be an issue if your bike is newer than 2015
104mm is for the 68mm shell, 109mm spindle is for the 73mm shell.