Cannondale has four level of stems...
C1 (lightest and strongest)
C2 7000 series aluminum
C3 6000 series alumnium
C4 (heaviest and weakest)
Let's take a quick look...
Thanks to prendrefeu's post on weightweenies forums and choiboi's post on roadbikereview forums.
Cannondale C3 100mm stem with dual arm face clamps and steel bolts: 114.6g
Cannondale C3 100mm stem with full face plate clamp and steel bolts: 132g
There is one difference between the C2 and C3 stems:
1. The C2 is made from 7000 series aluminum and the C3 with 6000 series aluminum. The 7000 series is stronger due to a higher proportion of metal in its properties but the strength overall between the two in cycling applications will not be noticeable to a rider if it's the frame or stem. Aluminum bikes are often made with 6000 or 7000 series aluminum and no one notices a difference in the ride or strength of the frame or parts.
Conclusion: The C3 with dual arms (not the full face plate) at $20 - $25 is a good stem, good weight, at an excellent price. It's even decent with the full faceplate which is about 9g heavier. I just received my stem, new stock and it weighed in at 141g as you can see below. So I think the stem above may have Titanium bolts which are nearly 10g less than the steel ones or it's a shorter stem.
C3 100mm full faceplate = 141g