The plastic seal insures you that the speedo is original (and originally connected) from the factory. As I remember from years back, it's flimsy and can be snapped off easily.

The "wing nut" collar (your description is good) turns just as if it had a normal right-hand thread. A quarter-turn should do the trick. Yes, it's stubborn.

If it snaps, you'll probably find that simply inserting the cable end into the speedo will work -- I drove my '81 245 about 50k miles without the stupid collar, and the new owner has put another 50k on it the same way, and it still goes fine today. The tension in the cable keeps it tightly engaged in the speedo.

--
Don Foster (near Cape Cod, MA)