The 24/26 wheel combination was the SM-series MTB, which continued until ~1987, including the RollerCam brakes.
Supposedly the smaller wheel enabled a shorter wheelbase , for better traction and maneuverability on the steep, technical trails of the eastern U.S, where they were based.

Yellow and (less frequently) Pink have featured off and on in the catalog (lots of yellow in the 90s) and I recall a berry-pink Road bike (a early ST-, maybe?) Keep in mind, that pre-1990, Cannondale was pretty much a boutique maker, and they also sold a lot of bikes as near-custom bare framesets, so it’s not uncommon to find one that doesn’t match up with the catalog.

SN# under the chainstay was the normal location for pre-1992. VintageCannondale.com has a pretty good database and SN# decoder to get you started