TEC 140 - 180 lens caps have two holes in them where your thumb and finger go in, then you squeeze the two squares (seen below) together, which releases the cap from the rounded front of the dewshield.

They are spring loaded, so when you put it on, there are two delrin-looking hook-a-ma-bobs (not seen in pic) on the back of the dewshield cap, which lock inside the reducing diameter of the rounded dewshield front.

You can see in front of the lens engraving ring, on the bottom, there is a groove seen. That is the area where the hook-a-ma-bob go into and then expand to lock.