Port and Starboard
This is a guide to help you in determining which Grill Stanchion you need. Firstly, let's get you up to speed on Port and Starboard. Simply stand in the boat facing forward. The letter "P" come before "S", so the LEFT side side of the boat is the PORT side. The right side of the boat is the STARBOARD side.
Port wine is a RED wine, so when you see a red light on the bow of a boat, you're looking at the PORT side of that boat!
Chaparral Vortex (-3 degree system)
If you have a Chaparral Boat, your only option is for the PORT side grill stanchion, as your mount is on the PORT side as below. I've been told that all the larger Chaparral's have a PORT side mount, and that the radio control is always on the starboard side. Smaller Chaparral's (below 23') don't have a mount from the factory.
Chaparral 243 Vortex with the PORT side mount.
Yamaha 21' Boats (12 degree system)
For the most part, it appears that the 21 foot boats from Yamaha have a TRANSOM mount. This is a mount that faces rearward, or the STERN of the boat.
Yamaha 212 Series with a Transom Mount
Yamaha 24' Boats (12 degree system)
The AR240 and 242 series boats come with mounts on either the port or starboard side.
Yamaha Port Side Mount
Yamaha Starboard Side Mount
Yamaha 27' Boats (12 degree system)
The model 275 only has one mount location, and it's on the port side. The model 275 stanchion is a bit different than the Model 242 stanchions, in that the 45 degree leg is longer, since the Model 275 has a larger swim platform.
Yamaha Model 275 Port Side Mount
One final reminder: Don't order a Chaparral port side mount for your Yamaha, or a Yamaha mount for your Scarab! The Yamaha mounts are unique in that the have a 15 degree spacer behind the mount on the boat, while the Chaparral and Scarab's do not. The mount itself has a '3 degree tilt to it (bottom of mount is further inboard than the top of the mount).
Hopefully, this clears things up for you!