Hi, I pulled the box of this model out of the closet and looked at the instructions on it see what you're referring to here. As I have yet to build this kit, anything that comes up here on this site about it, I jot down on the instructions for future reference.
Dragon's instructions are notorious for their inaccuracies and unside down drawings, as FAUST has pointed out in his post. And with the level of detailing on their kits, esp. those produced today, this is something totally unacceptable for the quality of their moldings. I don't understand why they do this at all.
I do have a couple books on the 251 AFV, but unfortunately at the moment they are buried out on the back porch, as I'm putting in a floor in the room where I normally keep them.
As you suggested, it does seem to be some kind of detail suspension component and may not mount up to anything on the end of D-21, as I do not see it shown on any of the other drawings. It may just 'rest' someway on one of the road wheel arms-D-20 as well, or not.
The end sure looks like it should go onto something through.
Hopefully somebody can give a more detailed and appropriate answer then me.
Thanks FAUST for the info. on step 3 here. I marked it down on the instructions for the future.
Take care, Sgirty