Here's a pic of my rig. The plastic piece goes between the Tach and the console so you can adjust the angle. I still need to look for a wiring diagram to see how it all goes. Like I said, My car did not have this factory tach. Got it from one of my swap meet buddies at Carlisle. Looks darn near NOS to me. Also, I'm not sure if my console plate has the knock out for the tach. I'm gonna pull the dash and the console next week, so I'll see.
Looks like you are missing the base. Also, I took my car apart during the build and I didn't have a sending unit. You sure that's the correct tach?
Here's some pics of my tach installed. You can see from the second pic that there's a base that goes between the tach and the top plate of the console. This is the correct tach as ordered on the car's broadcast sheet.